I have had AVC preamps here from Acoustic Imagery (Jay-Sho) and more recently, Pal Nagy’s icOn 4PRO, with four Slagle autoformers. I owned the PRO4 and had it for a more extended time but in both cases, I noticed a bit of sound that I found somewhat “unnatural” compared to the resistor based preamps I was used to. It was sort of a very slight sheen or slickness (hard to describe) that just seemed a little off. However, it may have simply been that I was used to resistor volume controls. In any event, I did not find it to be a deal breaker and in both cases, the AVC sound was transparent, organic, and displayed a touch of warmth or body. They were inviting to listen to and, in the absence of other really good sounding choices, I could have been happy with the PRO4 in my system. Because of the impedance matching benefit of the autoformers, the AVC preamps are one choice that can be run passively and not sound lightweight or tonally thin. In fact, running the output of the icOn PRO4 through my SMc Audio unity-gain buffer did not seem to noticeably improve the sound.
I look forward to trying the Hattor Autoformer Reference Preamp (balanced model with 4 autoformers) both as a stand-alone device directly into my amplifiers and as a passive volume control with the output routed through my unity-gain buffer. In the case of Hattor’s resistor based preamps, the buffer improves on the tone and body over simply connecting the output to my amps in passive mode. Their own internal op-amp based active stage (gain adjustable by the remote - very cool) improves on the passive output in some ways but the op-amps don’t match the sound quality of my SMc buffer. I have also owned the Hattor Tube Active Stage and it was a very nice sounding step up from the op-amp active stage, but was also not quite to the level of the SMc buffer, IMO. In general, it is my experience that to improving on Hattor preamplification options requires spending multiples of the price.