Well, some months down the line, you may if you've read this far, be wondering if it was all worth it? Does the amp work OK now?
Short answer: Yes. There may be a little more of a background hum out of the speakers when it is switched from Stand By to Operate than it produced before all the problems started, but I can't be sure. Anyway once it's cranked up the hum is not noticeable as it doesn't change with the volume settings.
Around the time I got it back I bought a Mexican Fender Strat and the single coil pickups come through so well. There is so much clear top-end. Ironically bearing in mind that it was the reverb that I initially had a problem with, I still can't get on with the Superfly's reverb sound even after the repair and $100 upgrade and so - gasp - I use a setting on my TC G-Major II that's in the effect loop.
I can only hope that the amp holds up over the years and doesn't let me down.