I used to play in a rehearsal studio in Manhattan through a Dumble Steel String Singer head back in the late 70's - early 80's. It was an amazing sounding amp that didn't require a boost or anything other than maybe a chorus pedal. I was in two bands at the time, an original punk band and a band that wanted to be Yes when it grew up. It fit for everything and may have been the best amp I've ever played through (tough to say though, it was a while ago and I love my Mark IV) I asked about buying it once and was told $4000 for the head. Now, this was when a '59 Les Paul could still be had for 10-12 thousand dollars. Mid 70's wine red Les Paul Custom with a DiMarzio PAF neck, DiMarzio Super Distortion bridge, straight into the amp. Bliss
I would say, if you really, REALLY want a Dumble, save up and get a Fuchs. It's 99% of what you get with a Dumble, and may be better to your ears as well. Dumble really just rewired a Fender amp and it sounded amazing, so to keep people from copying what he did (and what anyone could do with the instructions) he covered his changes in gobs of epoxy so no one would see his change to the original Fender's schematic. It's fair to say that people have guessed correctly over the years, there's only so many ways to rewire an amp and have it still work after it all.