What could be instead of side walls?

Ensure that speaker cables are connected in-phase. + to + and - to -.

I accidently connected incorrectly and my soundstage was all over the ceiling and inside my right ear.

And to top it off, I was playing Pink Floyd Dark Side of the Moon!!

My understanding is the non-parallel walls helps with eliminating standing waves, which is a good thing.
You didn't give enough other info though to completely address your problem.
Have you tried listening to them in a "nearfield" configuration?
Good luck!

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.

Although it's hard for me to understand your situation from your description without some clarification, I'm not sure I read it the same way as Rives. It reads to me as if both your speakers are actually quite far away, in your estimation, from both of the side walls. I'm getting that if you moved each speaker closer to the sides, they would become too far apart from each other in the middle, and if you moved the whole system closer toward one corner, you would have an asymmetrical left/right proximity imbalance regarding the side walls. You seem to ask if this is a good thing, or whether you should introduce closer side boundaries somehow. There also seems to be some possibility as to whether maybe your side walls are of unequal lengths or heights, or maybe aren't parallel.

In general, with the vast majority of typical direct-radiating box-type speakers, having a large distance from the speakers to the side walls would be a desirable set-up situation (as would equal distancing from the sides). If the distances are large (over 5-6 feet), then I doubt any asymmetricality would cause any problems (and non-parallel could actually help). If that describes your layout, then I would focus first on speaker separation and aiming, listening chair positioning, and any possibly interfering room furnishings between the speakers and the chair. Also, the floor should not be a bare acoustically reflective surface in front of the speakers, but covered with a rug or carpet, if it is not already. Your description does seem to indicate that your speakers are not too close to the front wall.

You should let us know what model the speakers are in this case, the size and shape of the room, what the treatments and furnishings near the speakers are, exactly how the speakers are positioned and aimed, relative to the room, the listener, and each other, and also the listener position relative to both the room and any large furnishings. Specific measurements and dimensions would be nice. Also, a more detailed description of the symptoms you are hearing would be helpful to us.