Subwoofer Reviews

I was fascinated by Herb Reichert's adventures with the KEF KC62 subwoofer, so I borrowed one. Beautifully engineered, contoured, and finished and chock-full of cutting-edge technology, it would be welcome in any room and easily integrated into any system. However, it struck me as not just small but miniaturized, like the meticulously functional samples made for the traveling salesmen of a century past. Since its two force-canceling 6.5" radiators were the same size as or smaller than the midrange drivers in my main speakers at the time, I had low expectations and returned it without comment. That was in 2021.

I asked for a pair of KEF KC92s in early 2024 in the hope that these two relatively small subs would improve my system. Like the KC62, the gloss white cube with radiused edges and white diaphragms is an aesthetic match to our redesigned room, and the KC92 ($1999.99) is chock-full of the same cutting-edge technology. There are two exceptions, according to KEF: "Uni-Core and the Smart Distortion Control Technology is only found in KC62, to overcome the issues found in very, very small cabinets. As KC92 has a larger cabinet, we are using two discrete drivers braced back-to-back." Still, the KC92 is very small compared to the huge subwoofers that are common in home theater systems, yet KEF claims a response down to 11Hz and a maximum SPL of 110dB! How? The KC92's "Music Integrity Engine" is a suite of custom DSP algorithms including "Intelligent Bass Extension" (iBX) to monitor signal level and content to modulate output level and frequency response so that the KC92 can play "louder and remain dynamic at any listening level." So KEF does acknowledge that "Hofmann's Iron Law" (footnote 1) can't be broken, but that they are, in effect, bending it.

Still, I figured the KC92s should be okay because in my room they were going to play music, not eruptions, earthquakes, nor artillery. Moreover, the surface area of each KC92 diaphragm is about twice that of those in my Blade Two Meta's woofers and they are designed, loaded, and powered for frequencies below 140Hz, while those in the Blades operate up into the lower midrange, around 450Hz. Furthermore, I was not looking to extend the LF range of my system downward; the Blades are quite satisfying in that regard, thank you. What I was chasing was elusive: Rich, full, taut low frequencies without the thumbprint of room acoustics.

I think most people do as I do and position loudspeakers to optimize broad-spectrum tonal balance and soundstage. In doing so, we do try to avoid options that are generally poor for low frequencies (eg, in corners or flat against a wall). Nonetheless, LF is not prioritized and so it is unlikely that the ideal loci for tonal balance, imaging, and soundstage will be ideal for LF, even though acoustical science offers us reliable geometrical data for LF performance. The use of separate subwoofers frees us to optimize placement of the low-frequency sources in the acoustic space, and the large wavelengths at crossover ease their integration with the main speakers. In fact, the employment of multiple LF sources, each interacting uniquely with room modes due to its placement, tends to flatten their overall frequency response with common signals below the transition "Schroeder" frequency (footnote 2).

The KC92s and the Blades are, of necessity, located in different positions with respect to both the horizontal and the vertical dimensions of my space. The KC92s are on the floor, while the Blades' woofers are 3–4' up and nearly halfway between floor and ceiling. So, in addition to providing additional bass energy and even before any consideration of DSP/room correction, this more distributed geometry should itself mitigate modal effects.

Well, that's my plan and I am sticking to it, but to get a full appreciation of the KC92s, I will also give them a run in some other scenarios.

KC92s with the KEF LS60 Wireless
I don't believe the KC92s were actually made for the LS60s, but they work together as if they did. Since there are RCA Sub outputs on both the Primary and Secondary LS60, you can connect a pair of KC92s or a single KC92 to either one. Then, the KEF app takes over and asks if you are using one or two KC92s and, if using two, it lets you choose to use them either as a stereo pair or in mono by sending both channels of bass to both subwoofers. It then guides you through all the setup parameters, including the high-pass filter for the sub, minimizing guesswork and eliminating the need to fiddle with the DIP switches on the sub(s). Then, you sit in your chair and tweak the settings using the KEF App, by ear or by eye with a microphone and a measurement tool (REW, OmniMic, XTZ Analyzer, etc.) or both.

That's what I did when I tried out the LS60s with the SVS SB-3000 subs I had on loan at the time of the reviewing in 2022. And my reactions, as far as I can re-imagine them, were much the same. Adding the KC92s, "the LS60s became monsters, their output capacity unleashed. ... Ease and transparency increased in the lower midrange, marginally but meaningfully." I also said that, with the SVS subs, "The low bass was perfectly integrated," but getting there was easier and more certain with the KC92s. The SVS boxes are larger, but their single 13" drivers are roughly equal in area to the sum of the KC92's dual 9" drivers, so it was not surprising that the KEFs were their equal in their ability to extend the bass of the LS60s with power and definition (footnote 3).

Take, for example, a recent recording of Prokofiev's Symphony No.3 (Gianandrea Noseda conducting the London Symphony Orchestra; 24/96 download, LSO Live LSO0391-D). I love this screaming, throbbing, surging repurposing of The Fiery Angel ballet, and it makes for great dynamic demands. Especially in the nervous and slithery third movement and the angry and pounding fourth movement, adding the KC92s to the LS60s let them deliver the overt emotionality without constraint and not for the faint of heart.

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Footnote 1: Josef Anton Hofmann (1924–2010) was the "H" in the KLH audio company. He defined three desirable parameters in speaker building—Bass Extension, Efficiency, and Small Enclosure—but demonstrated that you can optimize any one or two of them but only at the expense of the other(s). Thus, if you want a small enclosure and high audio output, you pay for it with limited bass extension. Similarly, if you want high output and extended bass, you better build it big!

Footnote 2: Below that frequency, room modes and standing waves dominate a source/speaker frequency response and, in most rooms, that will be from 100–250Hz, depending on room size. Above that frequency, room contributions become dominated by reflections.

Footnote 3: Since I was using only one input and one output for the LS60s, I connected the sub output from my Sennheiser Ambeo Plus soundbar to the other input on each KC92 and it did double duty for TV and movies.