Bass Balance Problems

I've been having some trouble getting the bass to balance out (left-right) in my listening room. As you can see from the picture attached below, my SBL's sit along the short axis and fire into the room (no alternative to this arrangement). I generally listen from the chair located in the center.

My problem is that once I have balanced the speakers for equal left-right output for most of the musical spectrum (which I do by simply adjusting the balance control while listening to a mono record--which, incidentally, requires that I rotate the control to the right some 40-50 degrees or so), the low notes are still quite a bit left of center. The left speaker seems to have much better bass reponse than the right one does. If I adjust the balance control further to the right to left-right balance the bass response, the rest of spectrum shifts too far right of center.

I'm pulling out my hair here trying to figure out what is wrong with this system. Naim has looked at the amp and pre-amp and they check out ok (though, i was told that equalibrium on the pre-amp was with the balance control a bit to the right--which certainly comports with my experience).

Also, I checked the silicone sealant on both speakers and they are perfect with no gaps. So, I guess I am down to the room. There is a huge opening to another room on the right hand side of the listening room. Could this be causing the bass from the right speaker to be falling off a bit? I thought not because most of the sound is direct from speaker to ear, no? The left of the room i damped by a couch (though lined with a big bay window).

Anyway, any insight into this would be greatly appreciated.

Judd