Another approach would be to use a variac and adjust the amp IP voltage to the point were the noise sets in. Then scope the V rails at the PS and pursue the noise from there. Maybe when he was using the DBT it was dropping the line voltage due to series R, and the hum was alleviated.
It was a team effort ! My technical knowledge and troubleshooting skills are still basic, but I'm progressing. I am happy to report a successful repair, hum is gone !
The amp is still on the bench running cheap testing speakers, but I think there will be sonic improvement over the old capacitors. This amp will get a complete re-cap over the next few years, this is the second wave of cap replacement, hopefully the next wave will be done before more hum sets in...
I took the risk of using NOS caps I had in storage for like 20 years, but bagged, boxed and in a cool controlled environment. Did a low-current "reforming" treatment on each for a day (rule of thumb is 1 hour reforming for each storage year). Measured capacitance before and after, as an experiment if I can use that NOS lot in future projects. But this is a stop-gap measure until I upgrade those caps with super-duper audiophile-approved quality parts after the rest of the overhaul is done.
Thanks to all, this forum is great!
Joris