Yeah, I'm not sure about the xxx. "Everybody's X", who is on this forum, has a XXX. Just find out where V1 is and swap with a new one. On a 5150, V1 is closest to the input jack. The farthest right tube. Since your clean works, you could find out which tube drives the clean channel, pull it and put in the gain position. If it's low volume on the gain channels, it's probably that V1 tube. 12ax7