Something you might want to do is swap the tubes around and see if your buzzing problem moves with a particular tube, Fender tubes are made for high gain just as much as any other brand name is, ( Fender or Mesa doesn't make their own tubes, they have them made to their specs and labeled with the company name) You'll see some tubes listed as high gain, but the 12ax7 design has a gain factor of 100, you have kind of hit on two factors that do make a difference, Noise, some brands make a high quality tube with very low noise specs, Tone, some tubes sound cleaner, some warmer, some fuzzy and dirty, it all comes down to personal taste and you might even want to use certain brands/models in diferrent channels to take advantage of that particular brand/models tone. I suggest you go to GT's website and do a little research, GT makes several different 12ax7's and they are widely available, I like GT and EH...........................