Hey-Hey!!!,
that power level is raght at the edge of what you can get from a SE 300B. It is well within what can be done with a SE Ultra-Linear rigged KT88.You'll probably want a bit more NFB, so consider E-Linear with something of moderate plate resistance like 6BQ7A's for amplifier valves. Two of them and a KT88 ought to give plenty of gain. Two stages right after each other is enough gain for a moving-magnet phono pre-amp, add another section and the E-linear driver, and you will have plenty I think. 5-6kOhm primary ought to alleviate the need for a lot of NFB, and less will give a smoother performance at overload.
I know a few Hi-Fi trix for this sort of thing, but a lot of them ought to work for Guitar work...Things like L-C filters and valve rectifiers( preferably DH like 5U4's and U52'). Anybody have an Hg-Vapour pair of diodes in their Guitar PS?
regards,
Douglas