Very fair point on rooms. As for placement of subs next to mains, that is good to know. I have almost never had our sub next to the main but in the opposite corner. Mainly due to living room configuration.IMO and IME it very much depends on your room size / dimensions and speaker type. The Arrakis' woofers, if memory serves act almost as subs, I think they cross around 100Hz. Also, Arnie's room was quite large so placement was not as critical as in most rooms with room modes. Putting subs right next to woofers in mains in most rooms is not the most ideal positioning as you're exacerbating the woofers' bass from essentially the same location.
That said, I admit I was actually thinking dual subs, 1 near/slightly behind each speaker...so glad to know about your point here. Will make sure to think about placement very carefully if we go thru the trouble of a major upgrade of the sub to the next level.