Background:

Oh yeah...humming...but not in time with the music. Sounds like 50/60 Hz mains hum, or very similar? The problem may arise "suddenly" or just kind of "creep up" on you.

Scenario 1:  The EHT unit is the original Quad ESL is starting get a bit tired, or the capacitors and diodes are, which means the diodes are not doing their job of screening the AC supplied at the bottom of the ladder (~590Vac- 610Vac). Sorry, you have to rebuild the EHT supply if this is going on. However, be sure to use a HT probe and DVM to check the 1.5kV and 6kV terminals, in case your problem is caused by...

Scenario 2:  There is continuous, high volume leakage from a bass panel.  This will cause the demand for current from the EHT block to reach a level it just cannot supply. So, in this case, you're off in search of a major panel leak, somewhere.  Look for purple "glow" in the dark around the panel.  This is a more advanced symptom of the same issue that cause panels to Hissssss.