The Mallory has been replaced below deck, but the bias filter quad 75uf looks to be original. After checking, tightening, searching for ground loops and judicious use of deoxit it still had the hum so I followed a couple tips Craig passed along, and ended up swapping the 7199's with a known good (and quiet) pair. Then balanced the outputs "by ear" and was able to reduce it to a barely discernable level. It's not "dead quiet" but the very next thing to it so I'm pleased with the result. It's nice not to hear that hum between tracks of music.
I have a Wavetek digital multimeter that performs some of CR functions, but it has limits. Is there a modestly priced LCR unit that I should look at?
Thanks for the interest, all of you, I appreciate the help and advice.