Well, about ten minutes ago I went to play my bass, and everything I played came out completely distorted/muddy! I don't have a distortion pedal, that's not it; I flatlined my EQ, it didn't help; I even took my BP200 out and plugged my bass right into my amp! What gives?! For reference: I play a warwick thumb BO through a fender bassman 60
And if what Munji said doesn't work, (but I hope that's all it is), you may have blown your Bassman 60 --- it wouldn't take much to do it on such a low-power amp if you are sending all kinds of weird frequencies through it with your BP pedal while the 60's power is near its limit. Fender's Authorized Dealer network is huge (and part of the Fender price). So, take advantage of it by taking the amp to such a dealer to have it checked out, if replacing the 9V battery isn't the answer.