Did you need to DC the 6SN7's heater? I have run 6SL7>6SN7>Finals and used AC heaters w/o hum. Just curious, as another part of the eq'n is that feeding a big cap input DC filter has usually made some other noise.

I've played with the phase splitter autoformer. I always go back to the balanced drive version with the LTP doing the phase splitting and a high leakage L choke. Maybe a better phase splitting choke is the key...:)

If multi-grid tubes didn't keep me so entertained, I'd go and try a PP DHT amp. Cathode driven...:) PP 6CK4>2A3>IT>845

maybe not-so-simple can be good too.
cheers,
Douglas