It might work... for a few minutes or so. But if you complain about replacing tubes, that ain't nothing when compared against replacing a power transformer.
The 6550 and 6L6 are plug compatible. However, where the 6L6 needs about 0.9 amps of current (each) for the heater, the 6550 requires about 1.6 amps of current (also each) just to warm up the heater. I would be really surprised if any power transformer in a Fender made amp would have almost twice the current capability in the heater windings.
It's your amp, you can gamble if you wish. I wouldn't. A competent tech could modify your amp for 6550... chances are he would add an aux heater transformer. If he doesn't... I suggest he show you the spec's on the heater current capability of the stock power transformer. I would want to see around 8.5 amps to be safe. (12AX7's draw about 0.3 amps on the heater)
You won't get any more power out of the amp with just a tube swap. That would require a power supply (B+) with greater current capability and quite possibly a different output transformer.
