Mag 3.6 Soundstage question....

approx 7 months old 3.6's

The question I have relates to the soundstage. In general, the speakers are wonderful. wide and deep soundstage very pleasing. However, I've noticed most specifically on some recordings that have slight increased sibilance, I have a strange occurrence. In the specific recording, a voice may be placed dead center. However, during "sibilant parts" of words, the "SSSS shifts to the left. The effect varies from recording to recording, overall balance seems fine. on soft recordings, it's not at all noticeable.

I assumed it was a room problem, and have since taken the task of fixing any possible reflections etc. Room is symmetrical rectangle, extremely dead at this point due to over damping for testing.

Speakers have been moved several feet in from left, right and back walls...same issue. Feeding a single left or right channel into both sides of amp as Mono does not fix the issue. image is dead center middle mono until "sibilant" passage.

sorry for long post...these speakers are so fine overall that this issue is driving me a bit crazy.

(I am in the process of having an audio savvy friend have a listen, just to make sure I'm not insane)

Thanks for any ideas anyone may have!!

PS, have contacted Magnepan directly, they are of course off to CES, so it'll be later next week before I can speak to a tech person there.