To tdog,
We're tweaking up Crack, S.E.X. 2.1 and Smack for RMAF right now. As we fine tune them (experimenting with more current in the Crack and we finally got around to putting a C4S in the S.E.X. 2.1), they get closer to each other sonically. That said, the single gain stage and the shunt regulator seem to give the Smack the advantage in resolution over the other amps. Where it comes up a bit short with respect to the other two amps is in gain. We were running a very marginal level from our DAC this week and we had to turn the Smack up all the way while the other two amps had some room left on the volume knob. With normal output levels it's not a problem. We adjusted the DAC output up a just few dB and that allowed the Smack to show its stuff with some room left to turn the knob- very quick and dynamic with anything from 32 ohm cans on up.
to Keto,
My inclination would be to try the circuit as a preamp without any transformer. The output impedance would still be OK with a lot of amps - though my own search has been for a preamp tube that will have an output impedance down under 1K ohm, without a transformer. My opinion is based upon trying the transformer output circuit as a preamp vs. a low output impedance tube with no transformer, last spring.