I agree with Subaruguru -- throw some blankets and draperies around the room and see if it helps. If it does, go for some room treatments. Best spots (without knowing the specifics) -- behind the speakers (especially the corners), in front of the speakers on the floor (not too close to the speakers), on the side walls between you and the speakers and centered on the wall behind you.
Also, cables can make a huge difference as Sean and others suggest (nice offer Sean!). Speaker cables may help a great deal (MIT come to mind -- see earlier threads on dark speaker cables). As others suggest, it's tough to jump to conclusions about the source of the issue without active experimentation. Good luck.