A reason to place stereo subs as close to mains as possible is the inevitable "leakage" of sound above the crossover level. On paper, a driver contributes 50% to overall spl at crossover frequency and a sub continues to "roll off" above from there.
Listen to your system with only the subs powered up, don't even need an spl meter.
All that "supra-sub" content from the subs would seem to affect imaging, etc. the least by such positioning, though I've not done subjective testing. My stereo subs will stay where they are - immediately adjacent to each main speaker.
A Velodyne SMS-1, Rives PARC and other of their ilk will do a reasonable job smoothing peaks to make the nulls less toublesome at one's listening position.
Listen to your system with only the subs powered up, don't even need an spl meter.
All that "supra-sub" content from the subs would seem to affect imaging, etc. the least by such positioning, though I've not done subjective testing. My stereo subs will stay where they are - immediately adjacent to each main speaker.
A Velodyne SMS-1, Rives PARC and other of their ilk will do a reasonable job smoothing peaks to make the nulls less toublesome at one's listening position.