Hi,
I've used the SUNN/Fender 1200s for eight years. There was no problem with the powersupply. Though there were problems with some solderings.
Are U shure U wanna use this heavy amp with an extra external poweramp?
I mean, the preampsection is not bad...
Just a hint: I do remember that the 1200S got a very lage circut board. I would check this circut board first and then exchange the onboard power supply. I think U should get it from fender.
Greetings
Oliver