NEVERMIND about the A/D recommendation below....just re-read your first post.
Don't know if this will work for you or not, but on page 19 of the manual, you might want to play with the A/D attenuator. I don't know how it works, but it seemed to work for my situation. I played around with my friend's XR50 to test out my speakers (towers), and everything was fine. However, when we hook up her bookshelves, the damn thing was turning off every 10 seconds or so. She spotted the A/D attenuation section in the manual, so we decided to try playing around with that setting. Forgot if it was on or off, but try the opposite of what you have now and see if that works for you. Good luck.
Not sure why it would sound the same as your other unit. Have you messed with the SPL calibration for the unit yet? Sorry if you've already addressed this in the post, but I wanted to be sure of what you mean by "everything's set correctly."