Eh... I don't know nearly as much about electronics as I'd like to.
I took my broken amp apart, hoping to find an exploded capacitor or something and found nothing obviously wrong.
I did find four potentiometers that are labeled
The "gains" do exactly what you'd expect them to do, which was an awesome discovery since it allowed me to dial out some distortion I was getting with high bass via 2v pre-amp outs to 4 ohm speakers.
I'm assuming CMRR is literal - common mode rejection ratio. The test car has an aftermarket stereo so the variables have changed, but I certainly have no way to evaluate any of them - so I didn't touch them. .
Interestingly, "LGAIN" controls the right speaker and "RGAIN" controls the left speaker. I guess Saab got the connector wrong at the factory. LOL.
I still don't know what causes them to fail, but maybe I will figure it out.
Thinking out loud, I might be interested in trying to re-use the amp enclosure with one of ST's many Class D amp chips. Amazon, Parts Express, ebay all have a huge selection of prefab boards and info about tuning & optimizing them are all over the internet.
What I can't help but wonder is if you have a $20 board and a $10-$20 power regulator ... is that better than a prefab amp you'd buy for $40... or worse?