Use the FX loop.
PLug into the amp like normal, and connect the FX send to some other amp. What does that FX send sound like coming out over there? Clean or distorted? And I think you tried this - plug the guitar into the FX return, is THAT clean or distorted?
These tests tell us if the problem is preamp or power amp.
And connect the amp to some other speaker. We could have a bad speaker here.