SWR WorkingPro 2x10c distortion issue

[wvjazzbass]

Hello friends,
I have an SWR workingpro 2x10c solid state combo amp. When running a signal into the standard input with all pots set at 12 o clock, the signal sounds very distorted. I have tried a basic troubleshoot, and it seems like the amp is not getting enough signal. With the gain maxed out the clip LED does not light even under heavy play. I tried running an effect processor through the effect loop with the same result. I have pulled the unit out and visually inspected the boards for obvious failures to no avail.
I am thinking it may be a preamp issue, but am really just taking a shot in the dark.

[Enzo]

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.