So my Ultra Plus is humming pretty loudly.
It's not like a feedback hiss, but a deep low hum.
With the master on 0 and all the channel volumes on 0, it still hums.
I've tapped on all the pre amp and power tubes, but none of them seem to do anything.
What could be the issue?
Tons of good deals on Harmony Central and Sevenstring.org.
Is this a new thing or has it always been like that? Could just be a noisy amp. Low-noise tubes might help, otherwise a simple noise gate or supression pedal.
WTLTL 2011
If it is the guitar and not the amp, then it is likely a grounding issue. Have it checked to make sure the ground is good.

The hum is constant, and it's not the guitar because it happens without the guitar even plugged in.
If I have the amp on, with the MV on 0, it still hums.
I've tried a gate, but that doesn't work.
Tons of good deals on Harmony Central and Sevenstring.org.
Hum with all the volumes down and no input usually means a power supply issue, since that's the farthest thing back in the chain, anything before that would go away by turning the volume down. You'll need to get that checked out by a tech.
There's not even a tech within a two hour radius of me.
Oh well, I'll have to figure something out.
Tons of good deals on Harmony Central and Sevenstring.org.
Try plugging it into an outlet that has nothing else plugged into it. Some devices (such as TVs) will make some 60-cycle hum on the circuit that they're on, and for most things that's either evened out by the power supply or it doesn't matter, but if your power supply is a bit quirky then that might be the issue.

If it's near a TV, laptop, or anything that generates a magnetic field (basically anything that has a magnet or battery in it) that will also make it hum.