I'm glad someone suggested to check the settings on the receiver. I would guess that something is off for regular two-channel listening. This happens sometimes with my preamp when I don't manually switch it back to the proper two-channel stereo setting.
Though I don't know how complex your receiver is, I'd suggest checking the setup on your system. See if the crossover for the speakers is correct and if they're set to small or large (if you're able to do this, of course). I'd run all the diagnostics on it and see if that helps first.
The second thing I'd do is make sure that your interconnects are decent, and not the basic wires that come with your CD player. I am not suggesting to go spend $1000 on new interconnects and speaker cable, but at least make sure you've got more than the bargain basement stuff. It does make a difference. Good luck.
