Balance control vs. Speaker placement

In my room, I measure the distance from each speaker to the exact center of the listening position to within about 1/8", and then fine-tune the exact distance by ear. That helps, as there's an optical illusion in my room that makes the right speaker look closer than it is.

In a previous room, I was getting the image pulled to one side despite all my careful measuring and wall treatments. By chance I found that a vertical cabinet edge well off to that side was the culprit. Sound was diffracting off the corner, which was acting as a secondary sound source. I would hang a little damping material on the cabinet when I listened, and the imaging was vastly improved (especially on close-miked vocals. I can't stand it when the sibilants come from a different point in space from the rest of the voice). You might have someting similar going on in your room - you might try draping a thick towel over various pieces of furniture to see if you can find the culprit.

Well assuming that doesn't help, I would suggest you use the balance control rather than positioning one speaker farther than the other, because you want to preserve the timing cues that would otherwise be skewed if you moved on speaker farther away.

I have moved the speaker to compensate for this and it works quite good. You only have to move it an inch or two so the side effects are small and better then an off center image. (at least to me) Ill also save you a ton of time and tell you that if the image in more on the right you adjust the left speaker and vice versa.

The alternative to a balance nob is a volume nob for each channel-slight mod.

I think the question was why one method is prefered over the other. The balance control mainly controls volumes on each channel except digital delay. Speaker placement controls both volume and time alignment. I'd suggest two simple tests:
1. Switch speaker cables at the amp to see if the favoring side remains.
2. Extream toe in to minimize room interaction. Starting with speaker facing directly at you or in front of you.

Did you try reversing your speaker cables? For almost a year my left channel was weaker than the right. I blamed the source, the processor, the interconnects, and the amp. Finally I bought an integrated amp and still the left channel was weak. That left me with the speaker cable doubting, so I reversed it. Its fine now. You might give it a try.