chungjh: Because Soundlabs are a line source, being close to the side walls shouldn’t be an issue.
I previously had my M1PX’s within a foot of the sidewalls in my 14-1/2 foot wide room with no center image issues.
Your placement specs seem to be well within reason.
Accurate toe-in should provide a very solid center image, as long as your listening chair is exactly between both speakers. If you haven’t, measure one speaker’s placement — corners of each speaker to the wall you say is “about 5 ft” behind the speakers — and adjust the other speaker’s to those dimensions, to make sure both are as exactly placed the same as possible.
Moving further back or closer shouldn’t affect the center image as long as your listening chair (& head) is Centered Exactly between the speakers.
It might be worth a shot to play a mono source to more accurately check center imaging, presuming both speakers are in-phase.
One speaker biased different than the other could affect center image. If you have a test CD with a center output individually for both right and left channel, and a decibel meter, you could check one speaker’s level against the other’s and adjust the bias accordingly. However, if you biased both speakers as the manual explains, the difference should be minimal. For the most part, bias should only need to be slightly tweaked. My bias pot’s positions have always been very close to each other’s — if one is at 12 o’clock, the other has always been very, very close to that. Thus, I wouldn’t expect large differences between one speaker’s bias pot’s position and the other, unless something is amiss with the speakers, pre-amp, amp, or very unlikely, perhaps the cables. -Mike