Something to check... Run your 3-Pro preamp out to your SWR input. If the sound is distorted, then you could have a bad preamp tube. If the sound is clean, then you have a problem in the 3-Pro power section which means that either it's time to replace a driver tube or the MOSFET bias needs to be adjusted. You can change out the tubes on your own, but if that doesn't fix it - you need to take it to a tech to have the MOSFET bias adjusted. It should only be a bench fee, and it's a simple adjustment. After a few years, it's not uncommon for the bias to drift and is perfectly normal to need to have it readjusted.
I play bass - not treble!
Official Ampeg Portaflex Club Member #76
Official Ampeg Club Member #511
Thunderbird Club Member #11