Got to think about this some more. In the meantime, go with Lee's suggestion on better feedback parts. Any imperfection in a feedback divider becomes part of the amplifier. You usually get the most improvement replacing high value carbon composition resistors with metal films, say anything above 20 kohms, but only if their noise contribution is actually being amplified. Bleeder resistors and such don't matter. Much less improvement replacing carbon films, which are pretty good to start with. My guess is the noise is coming from the tone amp, so changes in the power section may not accomplish much, but I've been wrong before. IMO, the impedances in the tone amp are all pretty high, so the hiss might be inherent in the design.