The REL Acoustics T/5x is the compact entry point into REL's celebrated T/x series, and it carries the same design philosophy that has made the British brand a fixture in high-end two-channel systems. REL has long argued that a subwoofer should not simply add bass to a movie soundtrack but should complete the loudspeaker, extending the response of the main speakers down into the lowest octaves while remaining fast, tuneful, and invisible when it is doing its job well. The T/5x is the smallest expression of that idea in the current lineup, sitting below the larger T/7x and T/9x models, and it is aimed squarely at listeners with stand-mount monitors, compact floorstanders, or rooms where a physically small enclosure is a requirement rather than a preference.
What follows is a deep look at how this subwoofer is built, why REL made the engineering choices it did, and what the published specifications tell us about its intended behavior.
Design and Topology
What kind of subwoofer is the T/5x?
The T/5x is an active closed-box subwoofer, meaning it houses its own amplifier and uses a sealed enclosure rather than a ported or passive radiator design. This distinction matters more than it might seem. A sealed box has no tuned port resonance, so it produces a more gradual low-frequency roll-off and, critically, superior transient behavior. The driver is loaded only by the trapped air behind it, which acts as a progressive spring. That air spring resists cone motion in a linear, well-damped way, so the driver stops moving almost as soon as the signal stops. For music, where the leading edge and decay of a bass note carry most of the emotional and rhythmic information, this tight, controlled behavior is the reason REL favors sealed alignments across the T/x range.
How does the driver work and why is it forward-firing?
The T/5x uses an 8 inch (200mm) long-throw, forward-firing driver, a size and orientation chosen to balance speed with output in a small cabinet. An 8 inch cone has relatively low moving mass compared to larger drivers, which allows it to accelerate and decelerate quickly, tracking the pitch and texture of bass lines with agility. The long-throw designation refers to the driver's ability to move a substantial linear distance in and out (its excursion), which is how a modest cone area still generates meaningful low-frequency energy. Because pressure output depends on the volume of air displaced, a smaller cone that travels farther can match the output of a larger cone that barely moves.
The forward-firing orientation places the driver on the front baffle, aimed into the room. REL uses this arrangement on several models to reinforce the sense of speed and to integrate cleanly with main speakers whose bass drivers also fire forward. It keeps the acoustic launch of the subwoofer and the mains in the same plane, which aids the seamless hand-off REL is known for.
Amplification and Power Supply
What amplifier does the T/5x use and how much power does it make?
The T/5x is driven by a Class D amplifier rated at 125 watts RMS. Class D is a switching amplifier topology, meaning the output devices operate as fast on-off switches rather than in the continuously conducting linear mode of a Class A or Class A/B design. The audio signal is encoded as a stream of pulses, amplified, and then filtered back into a continuous waveform. The practical benefit is very high efficiency, so far less energy is wasted as heat. That efficiency is what allows REL to deliver a genuinely useful power figure inside a cabinet this compact without the large heatsinks and thermal mass a linear amplifier of similar output would demand.
The 125 watt RMS rating is a continuous power figure, which is the honest way to specify amplifier capability because it describes sustained output rather than a brief peak. Paired with the low-mass 8 inch driver, this power is well matched to the enclosure. A sealed box is less efficient than a ported one in the lowest octave, so it needs adequate amplifier headroom to drive the cone to full excursion. The Class D amplifier supplies that current on demand while staying cool enough to live inside the small T/5x cabinet.
Why does the amplifier and power supply pairing matter in a small enclosure?
In a compact subwoofer the amplifier and its power supply must do more with less internal volume, and the Class D approach solves that space and heat problem elegantly. Because switching amplifiers run efficiently, the power supply feeding them does not need to dissipate large amounts of waste energy, and the whole electronics package can be kept dense and light. This is part of why the T/5x weighs only about 12.3 kg (27 lbs). That low mass is not a compromise. It reflects a driver of modest cone area, a sealed cabinet that does not require enormous bracing to control port resonances, and an efficient amplifier that avoids heavy transformers and heatsinks.
Build, Materials, and Finish
How big is the T/5x and where will it fit?
The T/5x measures approximately 300 x 260 x 314 mm (height by width by depth) including its feet and grille, giving it a footprint close to a cube and a form factor that disappears easily beside a rack, a sofa, or a speaker stand. These dimensions are one of the model's defining virtues. A subwoofer's placement flexibility is directly tied to its size, and a cabinet this small can be tucked into corners, slid along a front wall, or positioned close to a main speaker without dominating the room. For listeners in apartments or in dual-purpose living spaces, that compactness is often the deciding factor.
What finishes are available and why does the cabinet finish matter?
The T/5x is offered in high gloss black and high gloss white, both applied as a multi-layer lacquer that REL treats as a piece of furniture-grade finishing rather than an afterthought. The gloss finish is not purely cosmetic. A rigid, well-sealed cabinet is essential to a sealed subwoofer's performance because any panel flex or air leak degrades the pressurization that the closed-box alignment depends on. The lacquer sits on a dense enclosure engineered to stay quiet under the pressure loads generated by a long-throw driver, so the energy goes into moving air in the room rather than exciting the walls of the box.
Controls and Tuning
How do you dial in the T/5x to your system?
The T/5x provides three tuning controls: a continuously variable crossover, a gain (volume) control, and a phase switch, and together these let you match the subwoofer precisely to your main speakers and room. The continuously variable crossover sets the frequency above which the subwoofer stops contributing, letting you blend its output to the point where your main speakers naturally begin to roll off. Because it is continuous rather than stepped, you can find the exact overlap point rather than settling for the nearest fixed increment, which is central to achieving the seamless transition REL prizes.
What does the gain control do?
The gain control adjusts the subwoofer's output level so its loudness matches that of your main speakers. This is a level-matching task, not a bass boost. When set correctly, you should not be able to localize the subwoofer as a separate source. It simply fills in the foundation that the mains cannot produce, and the correct gain setting is usually lower than newcomers expect precisely because the goal is integration rather than emphasis.
Why is the phase switch important?
The phase switch aligns the timing of the subwoofer's output with that of the main speakers so their outputs sum constructively rather than partially cancel. Because the subwoofer and the mains are physically separated, sound waves from each arrive at the listening position at slightly different times. If they are out of phase in the crossover region, bass thins out. Flipping the phase switch and listening for the fuller, more solid result is the fastest way to confirm the subwoofer is reinforcing, not fighting, your speakers.
Connectivity and System Integration
What inputs does the T/5x offer and how do they differ?
The T/5x accepts three types of connection: a high-level Speakon input, a low-level RCA input, and a dedicated .1/LFE input, which covers essentially every integration scenario an enthusiast will encounter. The high-level Speakon connection is REL's signature approach and the one the company recommends for music. It takes its signal directly from your power amplifier's speaker terminals, so the subwoofer receives the same amplified signal, with the same sonic character and voicing, that your main speakers hear. This is why REL subwoofers so often sound like a natural extension of the system rather than a bolted-on device. The Speakon connector itself is a secure, high-current locking plug that makes this connection clean and reliable.
The low-level RCA input suits preamplifier outputs or sources with a line-level subwoofer feed, and the .1/LFE input is intended for the dedicated subwoofer channel of a home theater processor, where bass management is handled upstream and the subwoofer simply reproduces the low-frequency effects channel at full level.
Can the T/5x be used wirelessly?
Yes, the T/5x supports the REL Arrow wireless system, which is sold separately and transmits the signal to the subwoofer without a physical cable run. This is a practical benefit for placements where routing a wire across a room is difficult, such as a subwoofer positioned along a back wall or opposite the equipment rack. It preserves placement freedom, which for a subwoofer is one of the most powerful tools available for smoothing in-room response.
Interpreting the Measurements
What does the 32Hz extension figure actually tell us?
The T/5x is rated to reach 32Hz at -6dB in room, and that specification is more meaningful than a raw frequency number taken in isolation because of how it is qualified. The -6dB point is a standard way to describe the low-frequency limit at which output has dropped to half power relative to the passband. Stating it as an in-room figure acknowledges the real acoustic gain that room boundaries provide, which is how bass actually behaves where you listen rather than in an anechoic chamber. A 32Hz lower limit covers the fundamental of a four-string electric bass and the low reaches of most acoustic music, which aligns with the T/5x's role as a musical foundation for stand-mounts and compact floorstanders. It is an honest, conservative rating consistent with a sealed 8 inch design tuned for accuracy over the last-octave brute force that a larger REL model would provide.
Conclusion
The REL Acoustics T/5x is a study in doing a great deal with a small, carefully engineered package. Its sealed enclosure and low-mass 8 inch long-throw driver prioritize speed and pitch definition, its efficient Class D amplifier delivers 125 watts of usable power without demanding a large cabinet, and its full complement of inputs, headlined by the high-level Speakon connection, lets it integrate with two-channel and home theater systems alike. As the compact foundation of the T/x series, it is built for the listener who wants REL's musical, seamless approach to bass in a form that fits real rooms.