While I realize that you've indicated the boominess is present even at low volume levels, have you considered the possibility that at least part of the problem might be microphonics involving your source component(s), or possibly even both of the two amplifiers you've used.
I see in a post you made a couple of months ago that you are using, or at least were using at that time, an ARC CD7 as your source, which is replete with tubes. Is it by any chance placed between or close to the speakers?
Just a thought. Regards,
-- Al
Edit: Just after submitting this post, I saw the photos of the room to which you just provided a link. I think I may have had a point. Re your earlier mention of the problem being most pronounced on the left side of the room, I note that the left channel amplifier appears to be significantly closer to the corresponding speaker than the right channel amplifier.
I see in a post you made a couple of months ago that you are using, or at least were using at that time, an ARC CD7 as your source, which is replete with tubes. Is it by any chance placed between or close to the speakers?
Just a thought. Regards,
-- Al
Edit: Just after submitting this post, I saw the photos of the room to which you just provided a link. I think I may have had a point. Re your earlier mention of the problem being most pronounced on the left side of the room, I note that the left channel amplifier appears to be significantly closer to the corresponding speaker than the right channel amplifier.