I just picked up a very clean Korean made SVT Classic at a pawn shop for a great price and I need some suggestions to help clean up the tone. It's very fat sounding but the mids and highs are muddy sounding when connected to an older SVT 810. Is this a characteristic of the Classic? I've owned two other SVTs in past, a 70's and an 80's model and I'm not expecting this to sound as good. The bias lights on the back seem to indicate the amp is functioning properly. Would changing the preamp tubes to JJ or Rubys help clean up the mid and higher frequencies significantly ? What about the power tubes? Or do the the Classics just suck?