Yep, You can tweak the hum balance all you want.
Actually, I've made it into a habit to give it a little twist every now and then, it sometimes becomes inoperable over time if not used
Look up the schematic for that amp, You'll then understand how it works.
With some caps, the ripple can increase when the operational temperature is reached, so those may also be the source of Your problems.
The tubes however are the easiest to swap, so start with those.
PA100 is a great, unbelievably under-appreciated offering from Fender, it's a small wonder how they're as cheap as they tend to be. Over there in the US at least