The REL Acoustics S/550 sits in a category REL prefers to call a high performance subwoofer bass system, and understanding why begins with the way the cabinet, drivers, and amplifier were conceived as a single coherent instrument rather than a collection of specifications. This section examines the physical and acoustic architecture before we turn to the electronics and the connectivity that define how the S/550 lives inside a real system.
What type of enclosure does the S/550 use?
The S/550 uses a closed box, active design augmented by a passive radiator. This is a deliberate hybrid approach that combines the tight, controlled transient behavior of a sealed enclosure with the extended low frequency output that a radiating diaphragm can add. A pure sealed box rolls off gently and offers superb group delay characteristics, meaning bass notes start and stop with the source signal rather than lingering. By adding a tuned passive radiator, REL recovers some of the deep output that a fully sealed cabinet sacrifices, without the port noise, chuffing, or phase complications that a vented enclosure can introduce. The result is a system that behaves musically on transients while still reaching genuine subterranean territory.
How is the active driver constructed?
The active driver is a 10 inch (250mm) forward-firing long-throw alloy cone. The alloy cone material is chosen for its high stiffness to mass ratio, which keeps the diaphragm behaving as a rigid piston rather than flexing across its surface as it moves. Cone breakup, where the diaphragm stops moving as a unit and begins to deform, introduces distortion and coloration, so a stiffer cone pushes those problems well above the operating band. The long-throw designation refers to the extended linear excursion of the motor and suspension, allowing the cone to travel a considerable distance in and out while remaining under control. Long linear excursion is what allows a driver of modest diameter to displace enough air to reproduce low frequencies at high levels without compressing or distorting.
What does the passive radiator contribute?
The passive radiator is a 12 inch (300mm) down-firing diaphragm that reinforces the deepest output of the system. A passive radiator is essentially a driver without a motor: it has a cone and suspension but no voice coil or magnet driving it directly. Instead, it is driven by the pressure changes that the active 10 inch driver creates inside the sealed cabinet. Note that the passive radiator is physically larger than the active driver, which is a common and sensible strategy because the larger radiating area lets the passive diaphragm move less for the same air displacement, keeping it in its linear range. Its down-firing orientation couples that output toward the floor, which helps load the room evenly and reduces localization cues that would otherwise draw attention to the subwoofer as a discrete source.
Amplification and Power
The electronics package is where REL translates driver control theory into audible performance. The S/550 pairs a purpose built amplifier with the driver and radiator to keep the whole system tightly under command.
What amplifier topology powers the S/550?
The S/550 uses a Class D amplifier rated at 500 watts RMS. Class D amplification works by switching the output devices rapidly between fully on and fully off states, then filtering that high frequency switching waveform back into an analog audio signal. Because the output transistors spend very little time in their partially conducting, heat generating region, Class D achieves high efficiency and runs cool relative to its output. In a subwoofer this efficiency matters for two reasons. First, it allows substantial continuous power in a compact chassis without massive heatsinks. Second, the high current delivery and strong damping factor of a well implemented Class D stage grip the voice coil firmly, controlling the cone during both acceleration and the critical moment of stopping. The 500 watt RMS figure represents continuous power rather than a fleeting peak, so it reflects real sustained capability into the driver.
How deep does the S/550 actually play?
The S/550 extends down to approximately 21 Hz, measured at the -6dB point in room. Interpreting this specification correctly matters. The -6dB reference point is a conservative and honest way to state extension, because many products quote the -3dB point or an even more generous -10dB figure to make the number look lower. Reaching 21 Hz at -6dB means the S/550 is still producing meaningful, audible output at the very bottom of the range where organ pedal notes, synthesizer fundamentals, and the lowest cinematic effects live. The phrase in room is equally important: real rooms provide boundary reinforcement that lifts the deepest octaves, and REL states this figure in the environment where the subwoofer will actually be used rather than in an anechoic chamber that would flatter no one.
Connectivity and System Integration
Connectivity is the heart of REL's design philosophy, and the S/550 offers the full complement of connection methods that let it serve both two channel music systems and home theater installations simultaneously. This is the section experienced enthusiasts should study most closely, because how you connect an S/550 fundamentally determines how it performs.
What is the high-level Speakon input and why does REL favor it?
The S/550 provides a high-level input via a Neutrik Speakon connector, and this is REL's signature connection method for music. The high-level input takes its signal directly from the speaker terminals of your power amplifier, the same output that feeds your main loudspeakers. The logic is that the subwoofer then receives an identical signal to the one your speakers receive, including the sonic character imparted by your amplifier. This preserves timbral continuity between the main speakers and the sub, so the bass sounds like an extension of your system rather than a separate voice. The Neutrik Speakon connector is a locking, professional grade connector that provides a secure, high current connection and accepts all the amplifier output leads in one tidy termination. Because the high-level input presents a very high impedance to the amplifier, it draws essentially no power from it and does not affect the load your amplifier sees.
What is the low-level RCA input for?
The low-level RCA input accepts a line level signal, typically from a preamplifier output, an integrated amplifier with a dedicated subwoofer output, or a processor. This connection is useful when a high-level connection is impractical or when your source component offers a convenient line level feed. The signal here is at line level rather than speaker level, so it arrives before the main power amplification stage. Many users combine the high-level and low-level inputs across a two channel and home theater arrangement, and the S/550 is designed to accommodate that flexibility.
What is the .1/LFE input and how does it differ?
The .1/LFE input is a dedicated low-level connection for the low frequency effects channel from a home theater processor or receiver. In a surround system, the processor manages all bass management and sends a finished LFE signal to the subwoofer. When you use the .1/LFE input, the S/550's internal crossover is bypassed for that signal, because the processor has already performed the crossover and filtering work. This lets the surround processor take full authority over the low frequencies, which is exactly what a properly calibrated home theater system expects. Crucially, because the S/550 offers this input alongside the high-level and low-level inputs, a single subwoofer can be wired into your stereo system and your home theater system at the same time, serving both without recabling.
What does the balanced LFE XLR input offer?
The balanced LFE XLR input provides a balanced connection for the LFE signal, which is valuable in installations with long cable runs or where electrical noise is a concern. A balanced connection carries the signal on two conductors of opposite polarity plus a ground, and the receiving end rejects any interference that was picked up equally on both conductors. This common mode rejection keeps the signal clean over distance, making the XLR input the preferred choice in larger rooms, dedicated theaters, and rack based installations where the processor may sit far from the subwoofer.
Can the S/550 operate wirelessly?
Yes, the S/550 is compatible with the optional REL AirShip II wireless system. AirShip II removes the need for a physical cable run between your source and the subwoofer, transmitting the signal wirelessly to a receiver that connects to the S/550. This is particularly useful when the ideal acoustic placement for a subwoofer is nowhere near your electronics, a common situation given that bass performance depends heavily on room position. It is worth noting this capability is an optional addition rather than a built in feature of the S/550 itself.
Controls and Setup
The S/550 gives you the essential tools to integrate it precisely with your main speakers and your room. These controls are what turn a capable subwoofer into a seamless one.
How does the continuously variable crossover work?
The continuously variable crossover sets the frequency above which the S/550 stops producing output, allowing you to hand off smoothly to your main speakers. Because it is continuously variable rather than fixed at stepped increments, you can dial in the exact frequency where your main speakers begin to roll off, avoiding both a gap in the response and an overlap that would create a bloated midbass. Fine adjustment here is what makes the blend inaudible. Note that when the .1/LFE input is used, this crossover is bypassed so the processor governs the handoff.
What does the level control adjust?
The level, or gain, control sets how loud the S/550 plays relative to your main speakers. This is the single most consequential setting for a natural result. Set too high, the subwoofer announces itself and the music sounds bottom heavy; set correctly, you should not be able to localize the sub at all, only sense a fuller, more grounded foundation beneath the music. The goal is that the S/550 disappears as a discrete source while the soundstage gains weight and dimension.
What does the phase switch do?
The phase switch aligns the timing of the subwoofer's output with that of your main speakers. Depending on the physical distance between the sub and your speakers, their output can arrive at your listening position either in step or out of step, and when out of step the two sources partially cancel at the crossover region, thinning the bass. Flipping the phase switch lets you choose the setting that produces the fullest, most coherent bass at your seat. The correct position is simply whichever one gives you more output and better integration where you listen.
Physical Specifications and Finishes
What finishes are available?
The S/550 is offered in high gloss black and high gloss white. Both are lacquered piano style finishes applied to a high standard, allowing the subwoofer to either recede quietly into a room in black or serve as a clean visual statement in white.
What are the dimensions and weight?
The S/550 measures approximately 17.7 inches high by 14.75 inches wide by 18.0 inches deep and weighs approximately 46.3 lb (21 kg). This is a manageable footprint for the output on offer, and the substantial weight relative to the compact size reflects a well braced cabinet and serious driver hardware. A heavy, rigid enclosure resists the reaction forces of a hard working driver, keeping cabinet vibration from smearing the very bass the S/550 is trying to reproduce cleanly.