Found something and also got the amps both whisper quiet.
On Sunday I used my iphone crude decibel meter - and had 59 dB rebuilt amp and 34 dB old amp (meter at the woofer dust cover.) No HumX.
I tried a bunch of things - HumX, wz68 rectifier SS tube, only two output tubes in different locations, etc and in the end I did the following:
Two output tubes only - located in the rear row. Rectifier didn't matter - I'm using the GZ37. Bias at 500 mA. No HumX. Result: @35 dB. Silent! Now both amps are dead quiet - and I can live happily with this setup. I've always preferred the two output tube setup of these amps with my Klipsch (easier on the rectifier and leaves me tons of spare tubes and I can bias each tube individually.)
What I found when I first turned on the rebuilt amp and went through the various start up tests - was with two output tubes in the front row - I couldn't get the (facing) left tube to bias at 500 mA. The floor was 700 mA. The other tube was in the double digits for the floor so it was easy to bring up to 500 mA. With four output tubes it wasn't a problem as I was able to go from 700 mA to 1A - the recommended setting. And the amps play just fine.
Now with only two output tubes in the front row - I can't get the amp to bias - so I switched to the rear row - and now it bias' just fine. Both tubes floor is in the double digits for mA. Because of that - the two output tubes in the second row - I now have them biased at 500 mA - and the amp is dead silent.
The output tube is circled below that has the floor of 700mA:
This socket has the extension of the heater wires - that go to the driver board. In addition I also created the long heater wire bridge from V4 that routes along the chassis (this was "adviced" to me - though the person advising went silent - so who knows - I could have misunderstood him) - I'm wondering if the long twisted heater wires are contributing.
Anyhow - I'm perfectly content as is and don't really feel like messing with it any further - as who knows I might lose this nice peaceful setup. I think I should have simply kept the heater wires short and direct from V4 to V2 as everyone else does. Or perhaps this has nothing to do with it?
PS: this is how I orignialy had them wired - like everyone else:
Criticism I received was:
- Twists not up to the tube socket - leads too long untwisted.
- Heater wires over terminal strip and sensitive components - need to route along chassis.