Triaxis: Loud hum on Main out, quiet on FX send

Had this unit for 10y. Goes into 20:20 and Mesa 1x12 cab.
With a clean tone in Rythm Green and gain at 3, I get the following:
1. Very loud hum and buzz on Main Out (can't use it to record at all)
2. Extremely loud hum and buzz on Rec Out (can't even use it to play)
3. Dead quiet on FX Send Loop Out

I tapped each tube one by one and there was no microphonics.
Any idea what the problem could be?

Tapping might not show you the candidate that hums....pull out V5 and put in a tube that you definitely know that works flawless...

So I changed all the tubes (which included some NOS Mullards) and put brand new Mesa tubes. No difference. Also note that I use a Mesa 20:20 power amp with brand new tubes.
However, I made a breakthrough discovery which is that when the switch for FX send (no wired connections in/to FX send and returns in the back of the unit) is OFF then the hum and hiss is huge, whereas it is much lower when switched ON (green light on). I never paid attention to it before. So the noise situation is better although the TriAxis is not dead silent in Green Rhythm with Gain at 3 and treble at 7. Does this point to something abnormal with the unit?

Only other thing to mention is that I had bought the unit in the UK and had it modded for 120v upon moving back to the US. I don't know if the tech (forget who did it) might have done it improperly? Is this mod trivial or involved?

Thanks

May not be related , but I have a formula preamp that hums into a 20/20 pwr amp
either the main out or the rec out , I forget which as I took something else for the gig ,
it was a sevre ground type hum . the pre into other things is fine and other pres into the 20/20
is fine , I didn't bother to find out , maybe I'll look into it more .