Variac noise

The amp is 50-60hz

if by “bypass the variac” you mean nothing plugged in to it, then yes, and it’s completely silent.

Also possibly worth noting: when I power it on (amp plugged into it) it produces a very soft acceptable hum, but as the amp warms up over about 20 seconds, the hum starts taking on a buzzing quality.

-sorry, troubleshooting electronics is not something I’m good at…

So variac noise depends on load, as you report, once your amp warms up it hums more.

Unfortunately, hum isn’t very predictable. Several transformers from the same lot may all be the same, or there may be one that hums less and one that hums more.

Your friend may be lucky. Or he may have an amp that has a much lower load than yours. or his electrical system may be better. A couple of things to try:

1. Take your humming variac to his house and plug his amp into it and see if it hums.

2. borrow his variac for an evening and see if his variac hums at your house.

One thing people chase with transformer hum is DC offset. I’m surprised someone hasn’t already suggested that. I have never found DC offset to be the problem. But there is always a first time so maybe that is your problem. DC offset can be caused by unbalanced loads on your home system, but not likely.

You might find that a larger variac/transformer hums less since it will be less loaded. OTOH, sometimes a larger transformer hums more regardless of load.

A variac allows you to dial in any voltage. this is handy in a lab but is not needed in your case. there are many fixed transformers available that convert from 120v to 100v. The ones on amazon are easy to return if they hum. I would oversize the transformer by at least 100%.

Jerry

@ccogopher 

Again, variacs are not meant to be quiet. Your friend is lucky.

You could try a DC blocker on the AC line; clearly you're not interested in the method I outlined earlier, although it would solve the problem as well.

Not disinterested in your method. I’ve been trying to reach a tech friend who could try your idea. Also looking into a step down transformer as a possibility. 
I really appreciate your input as well as the others. 
thanks!