Well, I have had my Pre-Fender Bass 750 for 5 years. It has been in a rack since it was new. Never pulled out of the rack. I baby my stuff. In the last couple of weeks, I have noticed a pretty great decrease in power and in performance of the head. Could this be my Pre-Amp Tube going bad? Over the last 5 years we have averaged practice twice a week and played probably 300 shows. I always take the time to cool down the amp and I always carry my own gear.
I'd try replacing the 12AX7 tube. All tubes wear out over time. If that doesn't do the trick, there is a possibility of a blown output transistor. SWR usually uses multiple transistors in parallel to achieve the required power output. It's possible the amp can still run with some blown transistors. There is also the possibility of a power supply problem. An amp tech can check it out.
Could be you ears to , from playing to loud Seriously , try to replace your tubes. How long was it you changed tubes ?
