Are you thinking of a comparison between the SET 400 and 35 wpc Ultravalve?
The Fet Valve 400R Hybrid amp is no longer listed on the AVA site, the next amp in the lineup with tubes is the 600R Hybrid.
I have heard all three amps along with the Synergy 450 in different combinations, the SET 400 is definitely the sweet spot for AVA amps. We compared the Synergy 450 to the AVA Ultravalve Vacuum Tube Amp (35 watts per channel) on a pair of Nola KO speakers and at low volumes there was no appreciable difference. Of course, when bumping up the volume just a little, the tighter, deeper bass and unlimited headroom became obvious. I own both the SET 400 and Synergy 450, the sound is very close but the SET 400 is a little more balanced, smoother and disappears. The 600R doesn't have the warmth of the SET 400 so it may appear to have more upper midrange and treble but in reality probably doesn't, it definitely has greater headroom.
The Nola KO speakers are 90 dB, Martin Logan electrostatics are 91 dB so bass and headroom comparisons would be similar.
Just for fun I did compare my AVA SET 400 to the PS Audio Stellar M1200 monoblock amps which have 600 watts each and cost $5998 pr. I'm keeping the SET 400.
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