I have two suggestions for you...
The cheaper would be to try a Celestion speaker. I couldn't tell you which, but I have a few friends who use V30's (but they're 12"). They really warmed up their Fender amps.
The other option is to find a brownface voiced amp. I love my '63 Vibroverb RI which is a 2x10 rated at 40 watts. But is very useable at low volumes ad has one of the sweetest warm cleans you could imagine.
I believe that Allen makes a few similar in size and volume to your Princeton.