Agreed on the calibration.
You might also check that all the speakers are 'in phase'. That is to say that your speaker wire, being actually two wires, needs to match the positive on the amp side to the positive on the speaker side.
Another way to say it (assuming everything conforms to normal color standards,) is that the wire on the amp's red colored post always goes to the red post on the speaker. Same for black. Usually, speaker wire has some writing or a stripe on one side so you can keep track of which is the wire running from red vs. black.
Speakers will still work even if you wire one or more backwards but when your left speaker is wired 'out of phase' from your right speaker, it can produce a reduced sound quality. I'm not sure if it would be as blatant as the issue you describe but it is still something worth double checking.
