The REL Acoustics S/850 sits in the middle of REL's Serie S range, a family built around the idea that a subwoofer should complete a music system rather than simply add bulk to the bottom octaves. For the experienced listener, the S/850 is interesting precisely because it does not chase raw output at the expense of speed. Instead, it pairs a substantial 12 inch active driver with a 12 inch passive radiator and a Class A/B amplifier, then wraps the whole package in connectivity that is designed to integrate seamlessly with a two channel rig, a home theater processor, or both at once.
What kind of subwoofer is the S/850?
The S/850 is an active powered subwoofer positioned within REL's S line of high performance models. Active means the amplifier and driver live in the same cabinet, so there is no separate outboard amp to match or place. Powered means it draws from the wall rather than borrowing current from your main amplifier. The "S line high performance" designation situates it above REL's more affordable entry models and below the flagship offerings, which in practice means you get the more serious driver and cabinet engineering without stepping into cost-no-object territory.
Why does REL use both a driver and a passive radiator?
The S/850 combines a 12 inch forward-firing long-throw active driver with a 12 inch down-firing passive radiator to extend low frequency output without the port noise a vented enclosure can produce. The active driver is the motor of the system, physically pushing air in response to the amplified signal. The passive radiator is a second cone with no voice coil or magnet driving it; it is tuned by its own mass and the air pressure inside the sealed cabinet. As the active driver moves, it pressurizes the enclosure, and that pressure sets the passive radiator in motion. The benefit is a tuning that behaves somewhat like a ported design in terms of low end reinforcement, but without the chuffing or turbulence associated with a physical port. Firing the active driver forward while aiming the passive radiator downward also lets the passive cone couple to the floor, which supports even loading in the deepest registers.
What does the long-throw design accomplish?
A long-throw driver can move a greater linear distance in and out of the cabinet, which allows a 12 inch cone to displace more air and reach lower with authority. Displacement is the currency of bass output, and it is a product of cone area and excursion. Since the cone area is fixed at 12 inches, increasing the usable excursion is how REL extracts more low frequency energy from that diameter. The engineering challenge with long-throw drivers is keeping the motor linear across that longer travel so the cone does not distort as it approaches its limits. The tradeoff for the enthusiast to understand is that a well controlled long-throw driver gives you dynamic headroom for film effects and orchestral swells while still starting and stopping cleanly for musical detail.
Amplification and Power
Why does REL choose a Class A/B amplifier here?
The S/850 uses a Class A/B amplifier because REL prioritizes signal fidelity and a natural sense of timing over the theoretical efficiency advantages of switching designs. Class A/B combines a small always-on bias region, borrowed from Class A operation, with the greater efficiency of Class B for larger signal swings. In a subwoofer this matters because the amplifier's job is not only to be loud but to respond instantly to transients and then settle without overhang. REL has long favored the sonic character of Class A/B in its music oriented subwoofers, arguing that the transition behavior and low level linearity contribute to the tuneful, agile bass the company is known for. In listening terms, the goal is a subwoofer that reproduces the leading edge and decay of a plucked string or a kick drum rather than smearing them into a generalized rumble.
How does the amplifier relate to system integration?
Because the amplifier is matched inside the cabinet to the specific driver and passive radiator, you never have to guess whether an external power source is adequate. The internal amplifier is voiced together with the S/850's mechanical elements, so the crossover, gain, and phase controls all interact with a known quantity. This is a practical advantage during setup: you are adjusting a self contained system rather than trying to balance a passive sub against an amplifier of uncertain character.
Connectivity and System Integration
This is the heart of the S/850 for an experienced user, because REL's connection philosophy is what separates it from many subwoofers that offer only a single line input.
What does the high-level Speakon input do?
The high-level Speakon input is REL's signature connection method, taking a full range signal directly from your main amplifier's speaker terminals through a locking Speakon connector. The idea is that the subwoofer receives exactly the same signal your main speakers receive, including the sonic signature of your power amplifier. Because the connection taps the amplifier output at high level rather than a preamp output, the S/850 effectively continues the sound of your amplifier down into the bass, which promotes a coherent handoff between the mains and the sub. The Speakon connector itself locks positively into place, which is more secure than bare wire and keeps the high-level cable from working loose over time. For two channel listeners in particular, this is the connection REL considers foundational to proper integration.
What are the low-level RCA inputs for?
The low-level RCA inputs, provided as a left and right pair, accept a line level signal from a preamplifier or an AV processor's subwoofer or line output. This is the more conventional way to feed a subwoofer and is useful when a high-level connection is impractical, or when you want to drive the sub from a dedicated preamp output. Feeding both left and right lets the S/850 sum a stereo signal, which is helpful when a source does not provide a dedicated mono bass feed.
What is the .1/LFE input, and when should you use it?
The .1/LFE input is a dedicated connection for the low frequency effects channel produced by a home theater processor or receiver. In a surround system, the processor performs bass management and sends a single dedicated channel of low frequency content to the subwoofer. Routing that through the .1/LFE input keeps the movie soundtrack's bass on its own path so it does not interact with the crossover the same way a full range signal would. Many enthusiasts run the S/850 with both a high-level connection for music and the .1/LFE input for film, allowing the subwoofer to serve double duty in a combined stereo and theater room.
How does the balanced LFE XLR input help?
The balanced LFE XLR input accepts a balanced low frequency signal over an XLR connection, which is valuable in installations with long cable runs or in systems built around balanced electronics. A balanced connection carries the signal on two conductors of opposite polarity plus a shield, so noise picked up along the run is rejected at the input. For a subwoofer that may sit far from the processor in a larger room, the XLR LFE input provides a low noise, professional grade path that resists hum and interference.
Can the S/850 operate wirelessly?
Yes, the S/850 is compatible with the REL AirShip II and the HT-Air MKII wireless systems, both sold separately. These optional transmitters and receivers let you send the subwoofer signal across the room without running a physical cable, which is a genuine benefit when the ideal subwoofer position is nowhere near your electronics. The AirShip II is aimed at higher performance music oriented use, while the HT-Air MKII targets home theater setups. Because these are add on systems rather than built in radios, you choose the wireless solution that matches how you use the subwoofer.
Controls and Setup
What does the continuously variable crossover control do?
The continuously variable crossover sets the frequency above which the subwoofer stops contributing, letting you blend the S/850 with main speakers of any size. Being continuously variable rather than stepped means you can dial in the exact point where your mains begin to roll off, avoiding both a gap in the response and an overlap that would create a midbass hump. In practice you raise the crossover for smaller standmount speakers and lower it for larger floorstanders that already reach well into the bass.
What is the gain control for?
The gain control adjusts how much output the subwoofer produces relative to your main speakers, and it is the primary tool for balancing overall level. You set gain so the bass neither dominates nor disappears, matching the subwoofer's loudness to the rest of the system in your specific room. This is separate from the crossover because you can have the correct handoff frequency but still need more or less energy at that frequency.
How does the phase control affect integration?
The phase control aligns the timing of the subwoofer's output with your main speakers so their bass energy adds constructively rather than canceling. When the sub and mains are out of phase at the crossover region, their outputs partially subtract and you hear thin, weak bass. Adjusting phase compensates for the physical distance and positioning differences between the sub and the mains, and the correct setting is usually the one that produces the most bass at the listening seat.
What role does the level control play?
The level control provides fine adjustment of output, working alongside the gain to let you tune the subwoofer's contribution precisely to your room and taste. Having both gain and level gives you a coarse and fine approach to dialing in loudness, which is helpful when small changes make the difference between bass that supports the music and bass that draws attention to itself.
Cabinet, Finish, and Isolation
What finishes are available?
The S/850 is offered in high-gloss black and high-gloss white, both furniture grade lacquer finishes intended to look at home in a serious listening room. The gloss lacquer is applied in the tradition of fine cabinetry, so the subwoofer reads as a piece of audio furniture rather than a utilitarian black box.
Why does the S/850 include isolation feet?
The S/850 includes isolation feet to decouple the cabinet from the floor and manage how the subwoofer's energy transfers into the room. Because the passive radiator fires downward, the interface between the cabinet and the floor matters for both coupling and control of unwanted vibration. The included feet let you set up the subwoofer correctly from the start without sourcing aftermarket isolation.
How Should an Enthusiast Interpret the S/850?
Read as a whole, the S/850 is engineered for coherence rather than spectacle. The sealed-with-passive-radiator loading and Class A/B amplification favor speed and pitch definition, the long-throw 12 inch driver supplies the displacement for dynamics, and the connectivity is the real differentiator. The high-level Speakon connection is what allows the subwoofer to inherit the character of your amplifier, while the RCA, .1/LFE, and balanced XLR inputs make it equally at home in a home theater. With continuously variable crossover, gain, phase, and level controls, plus optional AirShip II or HT-Air MKII wireless, the S/850 is built to disappear into whatever system you already own.
