Dietz,
I appreciate your concern! I actually did some full-range sweeps, and apart from a couple of pretty noticable dips and peaks in the lower-mid, everything was pretty good -- as good as can be expected in a really small room. Imaging from normal stereo recordings, ranging from Boards of Canada, to Branford Marsalis, to Mahler are all consistent, and seem spot-on to me. The Aphex Twin track I was referring to must clearly have has some sort of virtual surround trick going on, as the sound makes a clear path all the way around the listener, in a continuous, slow circling motion. I did notice that one side is more pronouned than the other, though. There are two possible reasons for this:
1) there is a window in that corner
2) my ear on that side is visibly lower... yes, I'm a freak
I have a total of 5 high density rockwool panels installed. Two are in the front corners, two are above the speakers, covering the ceiling corner, and one is behind my head. The ones above the speakers overlap with the corner ones, so that the wall-wall-ceiling corner is also covered, and maintains about a 4-inch air space. The rest of the front wall (behind speakers) is treated with acoustic foam. I think I've done pretty well, actually.
Anyway, thanks again for the tip!
cheers,
J.