KEF R107 loudspeaker:
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KUBE stands for KEF Universal Bass Equalizer and takes the form of a separate solid-state component that connects in the tape monitor loop or between the preamp and power amp. The KUBE supplied with the 107 includes a speaker-specific equalization module that provides level equalization for the tweeter and mid driver, as well as an inverse of the 107's LF response. Furthermore, the KUBE allows the user to control several parameters not previously accessible. First, the LF cutoff is adjustable in four steps: 50, 35, 25, and 18Hz. Second, "Q" of the bass response is continuously variable from 0.3 (overdamped) to 0.7 (maximally flat). The Q parameter describes the sharpness of a speaker's bass resonance. As the Q increases, bass damping decreases until ultimately a response peak develops. The Q parameter also correlates well with bass transient behavior, high-Q designs resulting in underdamped, boomy bass with a lot of overhang. For example, jukebox bass may have a Q as high as 3. That's real, state-of-the-art boom.

The last control on the KUBE shelves the response below 160Hz up or down by as much as 3dB to suit one's tastes or room requirements. The bottom line (no pun intended) is that the KUBE offers extremely flexible control of the in-room bass quality, magnitude, and extension, that I for one appreciate very much.

The Case of the Sick Cube
Even KUBEs can get sick, and mine certainly wasn't well right out of the box, as I discovered when my Boulder 500 amplifier indicated "DC offset" and refused to play. I measured an abnormally hefty DC voltage of 700mV at both KUBE outputs. The day after I described the problem to Fred Yando at KEF America, I received a phone call from England. The KEF engineer with whom I spoke assured me that the KUBE parameters for my pair (SN 1407) were on file (footnote 2), and that a new KUBE would be on its way to me as soon as possible. A week later the new KUBE showed up at my doorstep via Federal Express, and I'm happy to report that I couldn't measure any DC offset this time around! KEF acquitted itself very well throughout this episode, providing efficient and courteous service. And, as Mr. Yando put it, KEF guarantees this level of support not just to reviewers, but to anyone who invests in the R107s.

Sonic Impressions
Whereas most dynamic speakers struggle to integrate their driver outputs into a semicoherent blend in the far field, the 107 is seamless. What struck me immediately about the 107s, when I heard them first under show conditions, was their wonderfully cohesive and effortless sound. That impression has not diminished in the comfort of my own home. The transition from the bass to the mids is magically integrated, and the conviction with which I perceive instrumental space and focus across the soundstage is heightened considerably.

Music emerges from the 107 like ripples in a clear mountain lake. There is no sense of strain or stress, even on very loud passages. The vocal-range magic of the Spendor BC1 and SP1 is present here in full force. Voice is reproduced very naturally, without any coloration. Neither are instrumental timbres slighted in any way, and harmonic accuracy is preserved up and down the audio bandwidth. String fundamentals and overtones are convincingly reproduced, and woodwinds possess excellent clarity and detail, while brass has its proper bite, without gratuitous brightness or harshness. With the right electronics, musical textures are fleshed out with a convincing mixture of soft and hard, sweet and brash.

This is not a hi-fi-ish sort of speaker; it doesn't overdo the music with impressive special effects or extraneous boom and sizzle. It is very enjoyable, always listenable, and soundstage imaging is excellent, with width and depth perspectives as good as anything I've had in the house. The illusion of a continuous soundstage independent of the speakers is very strong with the 107. Instrumental focus within this stage is very good; perhaps not as holographic as I've heard with the Apogees, but extremely good compared with the conventional competition.

There are two other prize-winning aspects to the 107. First, the bass. The quality and quantity of the bass octaves are exceptional—the best I've heard from a conventional full-range system. Bass power, impact, and clarity leave one breathless on orchestral spectaculars. Second, headroom is sufficient to duplicate, at a typical listening position (even in a large room), the full dynamic range of any orchestral work—even a Respighi. The 107 gracefully complies with the dynamic demands of the music—it's capable of blooming from soft to very loud without a trace of compression or distortion. For example, it is hard to think of choral music with more electrifying intensity and grandeur than Sir William Walton's Belshazzar's Feast (EMI SAN324). Andre Previn is able to get the most out of the London Symphony Orchestra, and the English baritone John Shirley-Quirk is particularly effective in the haunting "King's Feast" movement. This work demands a full-range system with excellent dynamics, and the 107 is equal to the task, handling the full orchestra and chorus with aplomb.

Midrange transparency, and resolution of low-level detail, are very good but not quite state-of-the-art; the Apogees and the really good full-range electrostatics better the 107 here. The Apogees in particular possess an outstanding transparency that I find addictive. These are areas where I believe conventional speakers will always lag behind high-tech designs. As with the Frieds reviewed in this issue, you'll need to juggle your priorities and decide for yourself.

Although the highs are very extended, they don't have the delicacy and speed of the best tweeters money can buy. I'm thinking specifically of the Plasmatronics helium-plasma design and the much older Dukane Ionovac tweeter. In fact, KEF's T33 tweeter doesn't even sound as good as the Celestion copper-dome tweeter in the SL600. This is most obvious in the detail of brushed cymbals: the 107 is very listenable up here, but unable to resolve all of the available transient detail with sufficient intertransient silence. There's a little more mud and less clarity with the T33. This really is the only significant reservation I have about the 107, and I hope that KEF pays the treble a bit more attention in future models.

The impedance of the 107 is an amplifier-friendly resistive load of 4 ohms, made possible by adding a "conjugate load" to the crossover, which compensates for inductive and capacitive impedances. However, the LF boost fed the amp by the KUBE will make life more difficult for tube amplifiers. And, although I had pretty good results with my modified Michaelson and Austin TVA-10, I suspect that such really top-notch solid-state amps as the Boulder 500 will work best with the 107.

Overall, I consider the KEF 107s to be an engineering marvel and, by a clear margin, the best full-range conventional speaker money can buy. This is a smart speaker (two brains, remember), so it's not surprising that—in my opinion—it blows away all other conventional speakers in our Class B recommendations. They are that good! I have enjoyed the 107 over the past several weeks, and urge you to audition them promptly at your nearest KEF dealer.

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Footnote 2: The use of the term "Reference" for this range of KEF loudspeakers means that both pair-matching and matching to the engineering prototype are exceptional; drivers, crossover components, and the complete loudspeaker are measured, and the details kept on file. Blow a driver and KEF will be able to supply you with a replacement that will match the sound of the original.—Dick Olsher