The original preamp used more traditional values on the first stage (100K plate resistor, 1.5K cathode resistor) and omitted the MOSFET source follower. It didn't give me quite as much crunch as I wanted, and the highs were a bit too muted. Adding the source follower allowed me to increase the plate resistor and thereby get more gain out of the first stage. It also lowered the impedance driving the tone stack, improving the high-frequency response. Now I get a nice bluesy crunch out of the preamp, and it sounds much better with humbuckers than it did before.
By making Vbias adjustable, one could easily add a bias adjustment even though the output stage is cathode-biased.
The output transformer is a Hammond 1609.
The negative feedback is currently disabled. It was an experiment to try to eliminate some annoying boominess in the speaker cabinet I was using. The negative feedback didn't make much of a difference, so I disabled it.
The phase-splitter design was shamelessly taken from Randall Aiken's web site.
The tone stack design was shamelessly taken from the ax84 P1.