I would say both rear and side wall distances are important, hard to say what room interaction will be, you really need to experiment.
For what it is worth, Nenad’s IB2s are away from side walls as far as mine, but his are much closer to the rear wall, and he does have some problems, so based on that you may see benefits if you go further from the rear wall.
This distance also impacts perceived soundstage depth, IB2s are incredible in relatively free space.
Bass was somewhat more boomy when I was driving IB2s with less powerful amps, including 4B SST, but it was not huge difference, there are many other aspects of 7B SST performance that I readily noticed as better, for example, the size of soundstage and imaging considerably improved, and there is a sense of effortlessness and openness to the sound. All these ranked much higher on the list of improvements when compared to bass performance. If I looked at bass improvements alone, I would question validity of going with 7B SST, but considering everything else, no way I would go back to 4B SST.
In regard to ceiling, I agree, it is very low, my ceiling is 9”, maybe more heavy ceiling acoustic treatment is needed in your case.
Nenad has been filling up his room with acoustic panels and tube traps, now he ordered TacT to see if it will make things better, instead of sinking money into more tube traps.
