usually the buzz heard in multi preamp stage distortion is caused by too much bottom end in the early gain stages. There are exceptions though. But for marshallesque type stacked channel preamps with a master volume, I usually sneak up on the bottom end using smaller coupling and bypass caps on the front end and going larger as I approach the PI. Because the signal is overdriving each successive grid you can still get plenty of bottom end by doing this. But too much bottom end in the early stages just squares the wave too much too soon. You can end up with an atonal buzzy mess.
If you get to the tone your after (ie: the right amount of definition, harmonic balance, note separation, etc.) and you still have a little buzz, you can try using top end bleeder circuits in the later gain stages like local NFB and/or signal caps to ground, both in small pf values. These often do almost as much harm to the tone as good. But I've never seen (or built) a really high gain preamp that didn't use them as a final resort for tone and stability.
Oh, also, at 20 watts (2x6V6s or EL84s) if your only getting buzz cranking this amp you may be hearing some crossover distortion in the power amp. This is especially so with EL84s. You can try reducing PI output and if that doesn't work check out the "Paul Ruby zener mod" on the AX84 site.