The center of the donut is outfitted with a 10" woofer on one side and a 10" passive radiator on the other and powered by a built-in 250-watt Class-D digital amplifier.
But there is a caveat: The KEF sub is designed to be used with receivers and processors that have a built-in crossover and a continuously variable level control. The subwoofer's level control is a three-position switch labeled 0 dB, +6 dB and +12 dB, which offers limited level control and it does not have a built-in crossover. The lack of these controls doesn't matter if the receiver includes them and many receivers have very sophisticated subwoofer controls anyway. It also lacks high-level speaker inputs often used with a satellite/subwoofer system. But, I don't recommend the KEF for use with systems that don't have LFE or low frequency crossovers and subwoofer level controls.
The advantages of a wireless subwoofer cannot be overlooked. The obvious benefit is the elimination of the subwoofer cable(s) from the receiver to the sub, often as long as 25' that snakes around the walls or across the floor. But the real plus is flexibility of placement. Arguably, a subwoofer is the most difficult speaker to place accurately in any room. Improper placement can lead to very lean or exceptionally heavy bass that can thwart a good listening experience and even a small difference in placement location can make a huge difference in sound quality. Placement options are greatly expanded when the subwoofer is freed from the receiver, allowing it to be placed where it sounds best.
Another plus is KEF provides an extra-long AC cord for the subwoofer, making the best placement even easier.
I ran the KEF through a few tests using some of the most dynamic bass content in my collection and I found it had surprisingly robust bass performance. It exceeded my expectations for a modestly sized 10" subwoofer. It rocked when needed and hit the low notes when required. The best example was Toy Matinee's "Remember My Name", a multichannel DTS CD with lots of rockin' bass and aggressive use of the kick drum. This disc (and track) is a tough test for any subwoofer and sounds especially good when played loud. The KEF sailed through the test, but my dog left the room when I turned it up.
"Dirty Laundry", one of the best tracks on The Eagles "Farewell Tour 1 from Melbourne" disc (DVD Video) in DTS 5.1 channel sound has a solid, pounding bass track that sounded clean and articulate on the KEF subwoofer.
The combination of the woofer and opposite-facing passive radiator helped distribute the bass evenly throughout the room. The two drivers working in conjunction prevent peaks and dips in the bass and result in smoother response.
On the other hand, the KEF offers solid bass performance and its interference-free wireless operation offers a lot of flexible placement options. In this area, I rate it 4.5 stars. Considering both factors and its $1,200 price tag, I'll split the difference and rate the KEF HTB2SE-W overall at 4.0 stars.