I've never touched the EQ settings -- they've always been flat. I've fiddled with the balance settings, and have gone from all-the-way left to all-the-way right, with the expected outcome when you don't have any left audio. It's currently set to dead center. (Hah... I must have had it set on dead left at some point!)

If anyone can chime with a set of components to check (have multimeter, but no logic analyzers, or anything else like that), and/or areas to check for loose solder connections, I can do that when I get home (provided that it's simple enough). If someone can say "oh, yeah... you need to replace XYZ component," I might be able to convince one of the hardware gurus here at work to help out with that.