I recently picked one of these up, the online videos don't do it justice, it's killer! Low input is fat Marshally crunch/grit with some bounce in the attack and the high input is semi VH4 ch3 tones with tons of dynamic response to lighter and heavier pick attack.
Free bump for a killer amp. I love my Big Max. I always describe it as: "...if Peter Diezel built a hot rodded 2204." It's got a lot more Marshall DNA and is a lot more open, with more prominent upper mids; but, it still has that Diezel gut punch in the low mids. That and the Einstein are my two favorite Diezels. Good luck!