Line6 AX2 Modeling Amp Emits Solid Tone When Switched On

[s.whalen]

I have an old Line6 AX2 I got from a friend. The only issue it had was with the master volume pot. It would crackle a bit around the "3" mark. One day after playing for an hour or so, I went to turn the master down all the way before switching the amp off, and as it passed through the area where it usually crackled, the speakers emitted a loud "pop" and then began emitting a solid tone. The tone didn't react at all to any adjustments of any of the pots or switches on the amp, and it didn't matter if the guitar was hooked up or not. I tried using different cables and guitars and it didn't change anything. The speakers themselves appear fine physically. I checked the fuse and it wasn't blown. Nothing seems or smells fried. I checked the master volume pot with a multimeter and it seems to be functioning normally. Any idea what the issue could be? I'd rather attempt to repair this myself instead of lugging it to a repair shop.