Having recently read a good portion of Aspen Pitman's "The Tube Amp Book", my thought would be to try some different 12ax7's in there.
Im pretty sure it has a tube in there. If so the gain of 12ax7's can vary greatly according to Mr Pitman. Being the founder of Groove Tubes, I think he's right.
My take away was that 12ax7's were designed for 100x gain, and in the good old days did that. Toady 80's is average, but from batch to batch can be much lower or much higher. On top of that several other tubes will fit that have different gain structures. Check out the book, or GT's site they may have a lot of the same info.