You will want to be no more than 39” or more than 7’ for smooth bass due to 1/4th wave rear wall cancelation.
I found this to be true for my Harbeth 40.3XDs, although I read somewhere 24" from the rear of the speaker so 39" from the front is a coincidence. The furthest I could get the front baffles from the rear wall was about 5ft - bass response was poor. 39" tightened up the bass nicely. I don't have side walls to content with.
You have a lot going on behind your speakers - which may be detrimental.