My Vox AD15VT has experienced a significant loss in volume. Just a minute ago, I was playing and the volume cut to like half of what it was originally. This had happened about a week ago so I just turned up the master volume (I may add that it never went back up so the aforementioned volume was already "messed up") but now the amp just won't get above a whisper. It still has a signal, just a very weak one. I'm pretty sure it's not the tube (the AD15VT is like a solid-state/tube hybrid) since it's just a preamp tube. I plan to take it in to my local music store tomorrow to get it "diagnosed" but I wanted an idea of what it could be beforehand. I have little knowledge of amps so I could use a lot of advice.