Signs of recapping

Didn't see this info in any posts....
Is there a way to detect if capacitors need replacing by the way the amp sounds?
My long dormant 410-130 sounded fine for a while but I've noticed it starts distorting when the master is at 10 and the volume is at 3-4. I'm not sure if it always starting distorting at that low a volume. The 20 year-old Ruby 34s tested OK when I resurrected the beast a few months ago. Could they just be failing or is it likely a capacitor problem?
Unrelated question...is it posible to replace the bright switch with a pot to dial up the brightness as needed?
As alway you guys are golden and I appreciate the advice.

It sounds more like the tubes themselves but if the caps are original, they should be changed. I usually see a loss in bass response first as caps go bad and getting what I describe as a "brittle" sound is common. Pesonally, I'd can both and start over. 20 yr old anything, much less rubies are going to be a crap shoot. You could put in a variable cap but I'd probably prefer a push pull pot with two dif caps. The old prescence control on the early fender amps was a separate animal. Mike