Okay need to do some retracing. Just taking pot shots...
You are sure the board swap results in hum to the other channel; granted..
Are you using this schematic;
https://www.hifiengine.com/manual_library/bk-components/ex-442.shtml
I get the left right deal... I see a crap load of 1.0uf and 100uf caps to gnd in the schema's.
heat sink getting hot.. to is too high idle bias. You can drive the bias adjustment transistor to no effect. like the outputs are set in and thats it with mosfets / many fet types so if cold and dead pot works cut it back a few mv's.
Stop using the variac! just the dbt.. and if the ps tests okay with the dbt don't need that unless in the ps has new components.
If warm up and still no bias idle adjustment? hmmm perhaps some bournes. might go ahead and replace those.
I don't have high batting avg.. admitted but I've hunted the 'wiley wabbit' many times and 'bugs' fudged me many times.