Interesting, while I certainly don;t have the expertise of either of you two I do have an old Fender Studio Bass amp which is apparently pretty much the same basic amp as the super twin, minus a distortion control and at 8ohms - it certainly has the same valve set up as you found in yours so someone may have used it as the basis for refurb'ing yours?
If you can get it working properly, and if it sounds anything like the Studio Bass, you will NEVER sell it. (although to be fair you will also never actually move it as mine weighs well over 60kgs)
Here's some info you might find interesting for what you're doing:
https://www.talkbass.com/threads/fen...s-club.809269/
http://johnkvintageguitars.homestead...tudioBass.html
(last link has the schematic if you want it)