I've been searching the boards, but haven't found a post with the same problem, so any help would be appreciated.

I've got a single 12" 8ohm speaker mounted in a cabinet, wired to a mono jack, then plugged into the 8ohm output of the amp with a speaker cable. With no instruments plugged into the amp and all volumes on all channels at 0, I get a loud hiss. With an instrument plugged in, the hiss is still there at any volume. I've tried replacing the speaker, replacing the wiring from the speaker to the mono jack, replacing the jack, replacing the cable from the speaker jack to the amp, and nothing helps. This problem has developed over the last few weeks, and gotten progressively worse. This amp does have an effects loop (preamp out, Power amp in) and two inputs. Not sure if it helps to know this or not, but throwing it out there.

Thanks for any suggestions...