I think it's most likely a room thing, even though you say it doesn't happen with other speakers and happens in other rooms. Perhaps the Soliliquy's have a radiation pattern that is susceptible to whatever the room problem is, and perhaps the other room you tried happens to have similar characteristics.
The most likely culprit is a reflection or diffraction to the side of the right speaker. Imagine you put a mirror up against the wall to the right of the speaker, located so that as you look at the mirror you see the tweeter. The mirror is at the first sidewall reflection point. Pay particular attention to absorbing or diffusing the sound at that point.
Another possibility is a diffracting surface between the speakers, like a widescreen TV or something.
Are there any diffracting discontinuities to the right of the right speaker? Like the edge of a bookshelf, or a table, or a door frame? If so, try covering the feature up with a towel and see if that helps. Also, is there a doorway or other "hole" behind the right speaker? That would also tend to pull the image in that direction. In that case, the fix requires a bit more experimentation.
You might make sure that there are no screw heads sticking up around the tweeters, just to eliminate that possibility. A screw head is small, but close to the tweeter it actually interacts with a sizeable portion of the tweeter's output.
One final question - do the sibilants - the "sssss" sounds - seem to come from the same point in space as the rest of the vocalist's voice? If the sibilants are off to one side, that will pull the image over. Possible causes are a high frequency imbalance, which you seem to have eliminated, and/or reflection or diffraction of the highs. If the two speakers sound the same (placed side-by-side with a mono source, switching back and forth) then it's most likely a room thing.
Good luck!
