I've got the very same model (maybe year too) and it has been very good to me for a long time now.
Not exactly a mod but based on my own amp and others I have worked on over the years it could probably use a comprehensive solder touch-up. On my own amp, knowing I would probably be keeping it & relying on it for a long time, I just retouched the solders 100% right after I bought it (used). Found lots of cracked ones along the way too.
Here again not a mod, but try experimenting with using the effects loop for outboard EQ, delay, and (gasp!) even distortion or booster effects. In my own rig I ended up running my entire floor pedal board with a simple 100K volume pot in a box at the tail end as a master volume. I found this eliminated the huge change in response when clicking in a distortion pedal in clean vs dirty channel selection. Of course I don't play extremely loud and never have the amp's clean or dirty volume controls above about "3" - anything higher overdrives the stomp box inputs.
Really I think the amps sound pretty good stock without anything in the loop (much better than the Hot Rod models) - just at a much higher volume than is appropriate for most club work.
The only thing that immediately comes to mind as a mod would be to convert to adjustable bias.