An extra 12AX7 tube and a tube driven effects loop. It has about double the gain circuit to pull off extra heavy stuff but it can also produce some really nice clean sounds as well.
Although my main band is country we have been adding some 80's rock here and there so we are now 40 percent classic rock, 10% 80's rock, 10% modern rock, and 40% country (most of which is modern country which sounds like watered down classic rock). We mic everywhere now and I wasn't able to crank my Blues jr up past the 9:00 position at our last gig due to stage volume... So I never got the output tubes warm so to speak. The idea I have is get an amp with about the same power and have it be my dirty amp and let the Fender stay more clean since I can't really crank it loud anyway. If I don't do that I'm going to get some type of 212 combo that is about 50 watts in power has a lot of bottom end (can we say Marshall) but if I do that I'll have to save longer.