Sound & silence. The SR551 had very low self noise on the wideband drivers. The EL84 had none. After a good work-day's play, the more compact stereo amp with its RFT transmitter pentodes goosed for 20wpc output by exploiting class A2 mode ran hotter than was comfortable to touch for more than 3sec. Even its metallic volume knob got warm. The EL84 monos were cool aristocats by contrast. Sonically, things were a lot closer than anticipated. The key difference had the single-ended amp slightly more fruity, moist and texturally loose. The push-pull alternative was a bit more dry and damped. This was purely relative. In absolute terms, the Cubes were anything but dry or damped. In the bass in fact, they were the dynamically more exuberant and potent.
On bandwidth, resolution and speed, these designs were rather equivalent, however. Presumably both run low but some negative feedback. On the THD board, the single-ended design seemed to score just a bit higher on 2nd order, the pencil pentodes on 3rd. Both produced a dense, dynamically lively, color-intense panorama as the antithesis of lean, bleached, nervy or ethereal. Having owned single-ended amps from Ancient, Art Audio and Woo and push-pull amps from Octave and Trafomatic plus reviewed many more, I thought the curtains on the difference window unusually drawn. Granted, this was on speakers which presented an ideal load for lower-power valve amps. It ran them well inside their window of lowest distortion, hence less coloration. Still, the German amp sounded gutsier and more robust than many a SET had whilst the Polish monos exhibited more of the lush life of single-ended than expected. This made looks, price and temps nearly more important deciders than sound at least on these speakers.
Given that, I'd favor the Cubes for running cooler; for using widely available tubes to roll various glass; and for needing a preamp to add requisite remote volume. On price and containing 4 inputs and a preamp stage, the Rike had the functional advantage but that forgets how each Cube is available with a 48-step attenuator. I'd simply asked for the pure power versions.
On cubic brawn, Artur reminded me that "I use them on my Sonus Faber Extrema. Our Polish dealer runs them with our own preCube into Diapason Dynamis. You really can try a lot of speakers, not just the widebander and high-sensitivity types." I already knew that the Cubes had no issue playing our Audio Physic 4-way Codex to room-filling levels. Compared to our €8'500/pr LinnenberG Liszt monos to stay on price, it was simply obvious. Control over the hidden 10" woofers in their ultra-crammed sub enclosures wasn't the 200-watt transistors' equal. Nobody in their right mind would have expected it either. It just served as reminder. 'Playing loud enough' isn't necessarily synonymous with 'uncompromised'. Here it's not primarily about high sensitivity specs. It's about filter complexity, associated phase angles and the instantaneous current draw of big woofers tracking gnarly bass transients. For most people, a 12-watt amp even on 88dB speakers will go plenty loud. If it were a simple two-way, it'd outperform a complex 94dB Stenheim any day of the week. On that score, our Codex then weren't 100% ideal. Kroma Audio's Mimí at 92dB meanwhile was perfect. So would be the inbound 94dB Living Voice R25A which Kevin Scott designed with Art Audio 300B SET in mind. Electrically, so was the already delivered 90dB Avantages Audio César with its minimum filter: 2-ways all. At 30dB below source unity gain on our actively buffered passive Wyred4Sound STP-SE MkII, the Spanish Mimí were my stand-in for Artur's Italians shown on the iMac screen for reference.
With Avantages Audio magnesium-alloy omnipolar super tweeters.
In this happy context, sensitivity just determined the amps' distortion magnitude through their power draw. Here the Cubes must have remained well within their 1% max THD window. On our upstairs 85dB Albedo Audio Aptica, I might have pushed them harder to perhaps hit 3% or more? These are arbitrary figures. They simply repaint our earlier picture of how 'loud enough' needn't mean 'ideal'; and why sufficient power headroom is always a good thing. It keeps matters in the sunniest zone of lowest distortion.
In insert, Diapason Dynamis 3-ways as an 89dB ported design reportedly driven very well by the Cubes.
To my ears, the Cube/Mimí combo was a golden ticket. Endowed with quicker reflexes than any 300B SET I'd ever heard except for David Berning's atypical Siegfried, it could bare some teeth when hard-hammered upper right-hand Latin piano, close-mic'd searing trumpet or vigorously bowed country fiddle should all exhibit authentic bite, not have the sharpness of their transients doused in tepid water. If the most massive attacks of mega drums compressed somewhat, they did so without any distress signal. Hence this wasn't noticeable unless one rigged up an A/B against our 200-watt Nord Acoustic class D nCore monos. Those present as unwaveringly massive, unyielding and slamming yet with that very same rigorously damped control also manage to dry up the artful fruitiness and poise of MySound's EL84. Those little bottles played it juicy and vibrant to prioritize strong colors, lusher textures and a more relaxed mien without that foot-on-the-brakes subtext which can accompany those qualities in lesser valve implementations.
