NWSoundman moded mine for me last year and did a great job. I think he still posts around these parts from time to time. Might want to see if you can PM him.
The most obvious is to replace the input transformer with something a little cleaner / more professional. Depending on your taste, that is -- if you like raw and dirty then by all means keep the original. In fact if you're in to raw and dirty sounds in general, then I wouldn't touch anything. I had one of my channels replaced with a Jensen.
You can also try swapping the output opamps for the burr-brown (Texas Instrument) equivalent. Then there are the capacitors -- I think just about any audio device could be upgraded by replacing any electrolytics with a higher-grade film type or similar.
