Great question. I have been looking at the icOn 4PRO passive preamplifier (that uses Slagle autoformers) as a possibility to provide remote volume control and a display in front of my solid state buffer. I am currently using a Hattor passive preamp in that role but I have been curious about the icOn and how the sound of an autoformer based volume control would compare to what I hear from the AMRG resistors in the Hattor.
I had an Acoustic Imagery Jay-Sho (autoformer) preamp a few years ago but found that using it alone (without a buffer) left me wishing for more weight behind the dynamics. To me, it sounded a little too smooth compared to what I was used to with resistor based units. However, using autoformers to control volume in front of an active buffer or low-gain active stage might be a design approach that would provide the best of both worlds. I would like to try the Jay-Sho again, or the icOn, in front of my SMc buffer or Hattor's Tube Active Stage.