Thank you, @erik_squires.
The ELACs have a high-pass/crossover setting (choice of 60 or 80hz, currently at 80) and also the option to “EQ” with a switch +1/0/-4dB for each of HF, MF and LF so I have set the right one at -4dB for LF to account for the corner placement. That way it’s not boomy.
As to your questions about wall/ceiling mounting and bass traps, I think you already know the answer 😂 The speakers are tolerated where they are. Previously I had one where the sub now is, but just off the cabinet, so I could sit in the other portion of the sofa and have both speakers closer together (current toe in is 45 degrees) and with no walls/corners directly behind. But that was deemed ungainly lol. And I am not moaning - this is our reception area for guests so it cannot be turned into a technical audiophile listening room. I am looking into “artistic looking” room treatments but with little hope. (NB: behind my current listening position is a window and there are two more on the right wall, so there it just makes things worse and prevents room treatment due to lack of surface, blinding the windows with Rockwool not being an option 😅)
I am a little puzzled by your opinion on second sub. Yes, it probably would require room correction by eg MiniDSP (between pre and subs), but presumably having two subs in “opposing” corners should help in smoothing out the general room response. Or are you just concerned that room shape + mains + 2 subs would be too complex? It seems these days a bit of REW + Dirac shouldn’t be too hard to manage, but I am inexperienced.