It really doesn't break up until you're really pushing the power amp stage and that usually starts at 6 ... then again I might get earlier breakup if I swap the tubes.
The good news is that it takes OD really well and you will get a good sound if you put a good quality OD in front of it. The 6V6 is enough to give you a dynamic sound at bedroom levels, but I don't think you're going to get power tube breakup.