EDIT: Biasing fixes the distortion problem. Read entire thread. I just recently acquired an Ampeg SVT-III Pro (oldest model). It is very dirty and gritty with lots of distortion, even at low volumes. I'm pretty sure it needs new tubes but correct me if I'm wrong. It currently has 3 Sovtek 12ax7-wa's in preamp and 1 Sovtek 12ax7-wa and 1 unknown 12au7 in driver tubes. I want a warn clean sound, but no distortion. I don't want any Chinese tubes. I've been looking into 12au7's as preamp tubes. Any suggestions or info would be greatly appreciated. Thanks, Nate.