I ended up purchasing the icOn 4PRO balanced unit, which has four Slagle autoformers (copper). It is supposed to arrive tomorrow and I look forward to trying it in my system, both as a stand-alone preamp and also as a volume control in front of my SMc unity-gain buffer.
I have been using a Hattor passive preamp (Amtrans AMRG resistors) in front of the SMc buffer for a pretty good sound, and recently tried swapping the Hattor for a Goldpoint passive (SMD resistors) I still have here. Both sound good with the Goldpoint providing a bit more precision and the Hattor providing a bit more body and smoothness - maybe due to the carbon film vs. SMD resistors?
An equipment manufacturer I know has stuck with resistive preamps and told me, in his opinion, "TVCs sounded excellent with simple material, but started getting a bit irritating and “confused” with complex, dynamic music. Ymmv." I notice he said "TVCs" and not "AVCs" so I don’t know whether there will be a difference between the TVC units he has heard and the AVC icOn unit. I also don’t know whether he used the units alone, or in front of an active buffer, or both, and whether that even makes a difference. I will give it a try.