Quicksilver Audio Horn Mono monoblock power amplifier:
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As its name implies, the Horn Mono is designed to match horn loudspeakers, but the important criterion is the speaker's sensitivity, not the use of horn loading. Sanders suggests sensitivity ratings in the upper 90dB range as being appropriate, which usually means horns. In any case, my own Avantgarde Unos have a sensitivity rating of 100dB (John Atkinson actually measured 102.5dB in the September 2000 review), so it seemed that the match with the Horn Mono would be a good one.

Setup
Not much to say here. Introducing the Horn Monos into my system was completely straightforward, the amps blessedly free of the glitches and malfunctions that have plagued some more esoteric—and much more expensive—amplifiers I've recently had for review (footnote 1). If there was a break-in period, it was either too short or too subtle for me to take much notice of it.

Placing the amps on the latest (1.2 MIB) Aurios component supports improved instrumental focus within the soundstage, but the effect was smaller than I'd heard with some other tube amplifiers, which suggests good vibration control. The amps sounded considerably better when getting AC through a PS Audio High Current Ultimate Outlet power-line conditioner and PS Audio Lab Cable power cord, so this is how I listened to them most of the time. The choice of interconnects between preamp and power amps had a significant impact on the tonal balance: the Nirvana SL sounded a bit too mellow; the Nordost Quattro Fils were a better match.

Sound
The Horn Monos' most immediately noticeable characteristic was their lack of noise. Every other amplifier I've used with the Avantgarde Uno horns has produced noise—varying from being almost inaudible from the listening position when music was not playing, to being audible during soft passages in the music. Like most horn owners, I've learned to tune out the noise, but the Horn Monos eliminated the need to do this. With the Horn Monos powering the speakers, there was simply no noise from the Unos at the listening position. I had to go right up to a speaker to hear any noise at all; even then, it was barely audible.

But there's more to amplifier performance than lack of noise, and the Horn Monos impressed in other ways as well. The sound was smooth, engaging, with little in the way of a distinctive characteristic that could be described as coloration or lack of faithfulness to the source. I find "accuracy" to be a troublesome concept when applied to audio equipment, in that it's difficult to predict the significance of different types and degrees of deviation from absolute accuracy for various individuals. Granted, if we could get reproduced sound that was a 100% accurate facsimile of the original, we would all want that—well, I think we'd all want that—but as long as reproduction falls short of 100% accuracy, there are varying preferences about the sorts of deviations from absolute accuracy that are acceptable. If components with 100% accuracy were available, or if there were no variations in the preferences for different sorts of deviation from 100% accuracy, then we'd all be buying amplifiers with the same general design approach and measured performance. We don't.

In the case of the Horn Monos, the sound, while not deviating from "accuracy" in any gross or obvious way, was in the classic tube amplifier tradition of being just a bit soft and forgiving rather than clinical or hyper-detailed. The sound was entirely comfortable, with an easy-on-the-ears quality that invited continued listening. It was also wide-ranging, with good bass extension, and highs that were perhaps very slightly on the soft side. I've heard deeper, better-controlled bass from solid-state amplifiers, and more airy treble from some other tube amplifiers (SETs like the Wavelength Gemini and the Cary CAD-2A3), but the Horn Monos were not far behind on any of these parameters. The soundstage was wide (monoblock amps come by channel separation honestly), with a very good sense of depth within the soundfield.

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Footnote 1: Late in the review period, one Horn Mono developed a pop about 30 seconds after turn-on—likely due to a tube warming up—but otherwise, this had no effect on the amplifier's sound.—Robert Deutsch