What I was doing with the amp:
I was running the amp like this: amp>DI box>cab
It's supposed to pass the load and work normally. I'm using a countryman type 10 which is a really expensive one and they recommend this use for it and the exact wiring I'm using.
It happened all of a sudden while I was using it: The sound is thin, all the low end is gone. There is a static in the background that is so high pitched I can barely hear it, but it hurts my ears(it's still subtle though, if you were standing about the speakers you wouldn't notice that the static was going on). Other than that it sounds like the amp is trying to work normally. The oddest thing is that the signal that goes through the DI and makes it into my interface is even stranger than the sound coming from the cab. It's thin just like the cab, but the meter floats at a certain DB even when I'm not playing and isn't audible in proportion to what is coming from the cab. Normally the meter will would drop to the bottom when I stop playing.
What I did:
I first plugged the amp straight into the cab to make sure that the DI wasn't the issue. I swapped all the power tubes at once and it didn't make any difference. I only have 2 12ax7s on hand so I swapped one at a time, and tested in between, and it didn't help in any socket.
I don't know what to do. I've never had a tube swap not fix an amp. The DI setup I was using it in is making me paranoid that something bad is wrong. I tested the DI with another amp and it appears to be working normally based on it not floating the meters alone.
Any ideas? Could more than one preamp tube have died at one time? Will dead preamp tubes even make it do that?
Worst case scenario: output transformer is cooked.
The amp wouldn't be working if you've cooked the output transformer - and you'd have been able to smell it.
Sounds like a tube issue of some kind - take it to a tech and ask for a complete once over and service