I have a 5-6 year old Ampeg SVT-3 Pro which I use probably every week for a few hours, and have gigged with a handful of times. Recently I've noticed that unless I let it warm up for a while (15-20mins) it sounds really muddy and causes some weird buzzy "distortion" in the speakers around F# and B on the E and A strings. I'm playing through a 410HLF. It's sorta hard to explain, but it seems to correct itself after a while. I don't have the bass or mids excessively boosted, though I do play with the treble pretty high. I don't remember it doing this until recently.
Should I get the amp checked out? Do I just need new tubes? Do they normally take a while to "warm up"?