I have an M60/VP150/QS8 config. I used an analog RS meter, (C weighting, slow) to measure pink noise on-axis and 15 deg (3 ft laterally) off-axis from the VP150 at a distance of 11 ft. Left or right 15 degrees off-axis, the output dropped about 2 db (about 70db to 68db).
Since my QS8s are only about 9' away and positioned at about 90 degrees, moving left or right 3' puts you significantly closer to one and further from another. In my room this effect outstrips the relatively minor VP150 off-axis effect. The sad truth is like stereo before it, surround is significantly optimized for a sweet spot. At least it's better than Quad
At least for HT it seems to matter less than music.
There are definitely complications in dealing with surround audio, but mine don't come from off-axis Axiom speakers behavior. Rather getting my sub calibrated when SACDs and DVDs evidently have two different bass output levels is a problem, as is doing two separate calibrations for digital Dolby sources vs 6 channel analog sources like SACD. Receiver or even calibration disc test tones don't seem to help much, as there doesn't seem to be any level standard for how SACDs (or even CDs) are recorded. After calibrating, I ultimately adjust the speaker and sub levels until it sounds right, which can vary from disc to disc.
