If you're not loving the breakup, the thing that helped me the most was getting the voltages down like in the older ones, an idea my tech had. Sounds and feels way better now. That was actually the thing that made me love that amp again, I was thinking about letting it go. It breaks up a bit earlier now as well.
Also, Fender famously biases their amps pretty cold as to prolong tube life. If you can get the bias up to spec according to the 6V6s you use you'll hear a change for the better as well.
But the GA Eminence speakers are great. I think I reported on my findings in this thread before, but there seem to be a couple Princeton-themed threads right now and I get confused.

Edit: It was this thread. Page 1.
