Stax has not made any transformers for their electrostatic phones for many years, going totally over to amplifiers. I myself have not had a transformer to run Stax phones in over 20 years having also gone totally over to amps.. But I obtained a low bias Stax SRD6 mains-operated transformer a little while back when I got an SR5 phone and finally got around to comparing the sound from the transformer versus an amp, mostly on an SRM1Mk2.
Whereas the low bias Sigma has a marked high frequency roll-off on the amp, I was immediately struck by the fact that the Sigma seemed to have a better tonal balance with the transformer. I am not sure why this would be. The amp I used to drive the transformer was an inexpensive Sherwood rated at 100 watts/channel, normally used to run speakers in the bedroom. While no world beater it was an improvement over the last amp I used in this setup. I suppose it's possible that the bias is out of spec either in the SRM1 or the SRD6.
However putting the Sigmas back into the SRM1 amp, I was struck by the fact that I was hearing much more detail and ambience.
Comparing my other low bias phones on these set-ups, I did not hear any advantge in tonal balance with the SRXMk3 or SR5 using the transformer. Possibly because the Sigma is so inefficient compared to these other phones, it needs a big power amp to bring it alive and that's what i am hearing.
Some users seem quite happy with their transformers but on balance I did not think that using this transformer was the way for me. But I wonder how a high bias transformer would sound. Would you get the same trade-off of balance vs detail and ambience? Also, would the benefits of a higher quality power amplifier could be heard through a transformer?