my yba 200 has been giving me alot of pick up hum so i figured i'd take the pre amp tubes out of my old fender guitar head and try it out on the traynor. i changed out the sovtek 12ax7's for the fender 12ax7s, but i don't know if i can change out the 12AU7 for the fender 12AT7. The amp has an auto biasing feature but i don't know enough about tubes to know if this is a safe change out. also, i don't know for sure but i think now i am actually getting more gain with the fender tubes which sounds good and i also believe they are distortion sooner which i like, but i don't know if thats just in my head. what i am asking is, is it safe to change out the sovtek 12au7 for the fender 12at7 (as the 'phase splitter') and, do the pre amp tubes really have alot to do with getting a different distorting sound or maybe even loosing basseyness or is it just in my head?