If no change, put the original tube back and move on to the next preamp tube, and repeat. So, for example, pull one of the preamp tubes and put a new one in. If you want to start the dx process and have access to spare tubes (maybe a bandmate's if you don't have a spare set) you might replace them one by one in a systematic way and see what happens. When a working amp suddenly stops working right, it's almost always a bad tube. I am not an expert, but I do play around with amp diagnosis. Good luck, I hope it's nothing big/expensive. I hope that helps some, at least to narrow down where your problem is. It's a solid state amp, but it's still the same concept. I bypass the preamp of the bass amp because I'm using a signal into dirt, then into a IVP preamp pedal, which makes the amp's preamp redundant - plus that could cause the amp's preamp to blow or damage it. I do that constantly with bass, but I haven't tried it with guitar. If the power stage still works normally, the problem is your preamp if not, chances are it's in your power stage. One way I would try to test it, is if you have an effects loop in the amp, plug your guitar into some kind of a pedal preamp only (or I think a good booster will also work), into the receive of the loop to bypass the preamp. It could be that the pre-amp is the problem and isn't giving the power stage enough to work with, but it seems like it would still get something a little louder as you turn the volume up from the available preamp signal level. Would a transformer do something like that and still work? If you swapped out the tubes, that only seems to leave something else in the line - plate, transformer, capacitors or whatever. I'm no expert by any means, but that sounds to me like something in the power stage.
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