Have you applied any power to the amp yet?
If so, It's a next time thing. No worries. The resistor replaces the B+ wire from the rectifier to the first filter cap. Some guys go as small as 1 watt, I use 5 or more, just because I have them lying around. Doesn't have to be 100K. Some have used as little as 22K, and I've used well over 100K. There can be other considerations....
A variac with diodes can totally work, but you're guessing either way, when you turn the dial. The resistor method may only take an hour or 2, and I know of 1 guy that says he saved one...after 36 hours. You just leave it on and wait for the voltage across it to get "close" to zero. The cap decides if it'll come back to life .... not you.
Gary's tube base/converted to diode solution is a nice plug and play one. I'd do it. Just not on a "time piece" amp, where having the original components can nearly double an amps value.(this is especially true with guitar amps that wind up in the $5K+ range, we're weird like that, and we'll pay through the nose for the original components.) You can learn an awful lot about saving caps on guitar amp forums, because stupid $$$ rides on doing just that. The "diode base" is a good all around tool to have in your arsenal, anyway. Assuming your rectification isn't a 5Y3(don't do it), and say, a GZ34? You can nudge your B+ a wee bit(add headroom), and hear what it sounds like with less sag.. You may hear the difference when you push the amp to the brink...
I thought it was fair to address your initial variac assumptions(as well as others). All you really needed was a resistor.(and some basic considerations) I used my variac "wrong?" for years, until someone gave a heads up. If you've already applied power?? I really don't think it's fair to you that we walk down this road in your post.... That's a 20 page circus...
If you haven't applied power.. That's different....We might be able to save your caps.. Otherwise, we should get back to your build(or at least post turn up procedure, elsewhere).
FWIW, Nerdvana? deserves a big ole hat tip, for stepping up to show you what he knows. Hands on is always priceless. Times 10 for the face to face.
-Bob