Just an idea, not knowing if this is a head or combo, regardless, start with one known good speaker cable to a known good speaker cab, a known good guitar cord to your guitar. It could be a list of problems, but if you start with the basics, you can go from there. Switch through the channels, note any differences.
Sometimes, from my experience, I would find out the simple connections I make over and over again gets mixed up.
The human factor I guess, well along with the aging factor in my case.
Give it a shot at low volumes but if there is something wrong with it, the less you run it the odds of making it worse
lessen.
/cheers