Troubleshooting by Tube Substitution a Fender Twin Reverb

[Jazz P Bass]

Glad that you got it fixed!
As an aside, I wouldn't be so quick to dis the hum balance pot.
After all, it is only a variable resistor.
The normal "fixed" hum balance is a 100 ohm resistor off each winding to ground.
The balance pot can vary the two resistances. A little more one way. A little less the other way.
The balance pot can help if the transformer windings themselves are not equal.
You may have had a corrupt pot.

[64deluxe]

It's just a variable resistor, but it can fail in ways that are, as I have just experienced, difficult to detect even with wise counsel.

I have never heard a Fender amp with a virtual centre-tap, and correctly wired heaters, make any noise, but my first hum balance pot consumed 20 hours searching for a wicked hum. First time I run into one with unbalanced transformer windings I may change my tune.

But this amp sounds better without it. Perfect in fact. That's what really matters.