Firstly, I did read Bob's short troubleshooting guide...
I have pretty much all-original Dynaco ST-70 amp. I replaced the two coupling capacitors at the bottom and the multi-cap with the new higher-voltage higher-capacitance equivalent from Dynakitparts.
Originally, previous owner "modded" the amp to be in triode mode by installing 100-Ohm resistors between pins 3 and 4 of all power tubes and disconnecting the power wire from pin 4. This way, one of the channels would go into oscillation (crazy high-frequency whistle), unless preamp was hooked up to it. So looks like some kind of grounding problem. Anyway, going back to standard/original pentode configuration fixed the issue.
Now, the amp works, but I can hear noticeable main transformer hum through the speakers (when the music is not playing, but I am sure it's obviously still there when the signal is on).
I installed the new 3-prong power cord, but it did not change anything (i.e. grounding the case with the third prong). I tried a different set of tubes (from my other ST-70, which is silent), but nothing changed. The power tubes are properly balanced too.
So is it a grounding issue somewhere in the amp? Where should I start? Thank you!