If I understand you correct, it's not a parrallel issue, but rather one where the walls or different distance from the speakers. You are probably getting an imbalance that is frequency dependent from one wall to the other. This is a difficult situation to rectify and requires that you measure the response of each speaker independently at the listening position and then closer to the adjacent wall. This difference shows the wall effect to a degree. What you will typically notice is a band range that is accentuated. I would guess that the bands on either side are different. As music changes emphasis on different frequency ranges the music sounds like it's dancing around the room. Now how to correct the problem can be very difficult indeed--situations like this are one of the reasons we offer consulting services. But in general you need to attenuate the accentuated bands on either side of the room to get a good balance back. The really difficult part is determining how much attenuation and what material is the most effective for that band width. The most effective solution is likely a combination of absorption and diffusion, but it's hard to say without room layouts and actual measurements.
