Come on, hewo, you've been around long enough, tell everyone it is a KUSTOM TUBE 12CRATE VC508, unless it is something else, and put up a schematic so everyone can see what you are talking about:
http://bmamps.com/Schematics/crate/Crate_VC-508_(07S251)_Schematic.pdf
You surmise changing the cap increased the hum, then only as a parting thought you mention changing pot values. What else did you change? Not sure what the new cap was going to do, but as DC it won't radiate much from its wires, or pick up anything. In post #3 you add you apparently changed the op amps. Anything else?
What does this refer to?
Pot values changed? Oh dear... Look at the first stage, gain pot P3 is 250k, and combines with 2.2k R5, that ration sets the gain of the op amp. By quadrupling that value you upset the designed gain of the stage. And later, the volume control P1 is part of a voltage divider comprising R6, R11, and the tone circuits, plus P1. Looking just at R6 and P1, they form a 10/1 voltage reduction in signal level leaving the tone controls. If you made p1 a 1 meg then you have reversed it to a 1/10 voltage reduction of signal.
So looking at the circuit, the output from P1 is meant to be a line level signal, about 1 volt. The signal is shown as 30v p-p under test conditions at the preceding plate, so with a 1 meg pot, instead of about a volt all the way up, your signal level would be a lot closer to that 30v level. That would seriously overdrive anything in the line out jack, as well as hugely overdrive the following grid. Note the plate of that following triode only expects about 10v p-p, but if you start with more than that at the grid, it won't be happy.
So it is vastly different from changing the pots on a guitar. They affect impedance, but not overall signal levels or gain.
So what you did was increase gain in the first stage, but also increased signal level at the master volume. I don't doubt the amp is not stable.