New to bass guitars and amps. Just bought a used Fender Rumble 100. It makes a really odd buzzing when playing. Thought it was the Piezo horn. It's not--I disconnected it and the buzzing it still there. There are NO visible tears in the speaker itself. But the amp almost seems like it's not as loud as I'd imagine it to be, though it's still relatively loud. The seller is looking for the original receipt, so I could try a Fender warranty repair (5yr transferrable warranty). But I wonder if I should just try replacing the speaker itself? How much would that run, for a decent 15" speaker? Doesn't have to be top-of-the-line, but needs to be decent. HELP?