If the amp has a line out, does it follow that it also has a line in/power amp in jack? If so, it might be a switching jack, and it might be that its "switch terminals" are corroded. I've had low output/raspy distortion from older SS Peaveys before that went away if I put a patch cable from the line out jack to the power amp in jack, thus breaking that switching connection and replacing it with the patch cable's conductors.