You wouldn't have the tape monitor flipped on, would you? I spent ten minutes puzzling over my new KR-6030 before I realized that was the problem. Silly little thing, I know, but these are the sorts of things to check before determining there's an issue requiring service.
EDIT: Beaten to it, as usual. You might be able to tell if there's an errant setting by hooking up a set of headphones, selecting the tuner, and turning the volume up full blast to see if you can detect any sound bleeding through (a flaw, but a useful one, a lot of equipment has). I don't think I need to tell you NOT to hit any settings until after you've returned the volume to a reasonable level, lest you get an ear-full.