@dyson2004 Wow, more power!?! Those D3's aren't particularly hard to drive at 8ohms and 89db/w@1m. If they were more like 83ish, then you'd need approximately twice the power to be at the same levels you have now. But no problem for an AVR850 to drive whatsoever.
My lounge area is 10 1/2ft x 14ft. I use an AVR550 into a pair of GB1s augmented by a sub, plus centre and rears. It's carpeted with soft fabric furnishings, so good absorbers. However it's part of an open plan space which combined is larger than twice that area. I don't have any power issues from the AVR550 to fill the room with a performance that's too loud for comfortable listening in either stereo or surround. Only my sub has trouble keeping up. So do you need more power in your room? No!
I'm going to hazard a guess that your ceiling is approximately 9ft high? Is that true? In which case you have a room incidental you cannot do anything about.
What you might find is a different problem with those enormous speakers placed in a room of that size, and that is they might be starved of space to breathe. I'm not kidding here, but there is such a thing as too big! Couple that with an incidental and you're multiplying the problem.
So what's the trick? Don't bother with wall based room treatments to cure incidentals, that's a complete and utter waste of time. Smaller speakers that you can place in open space with a very carefully placed listening position could make a big difference. Soft furnishings are going to be the only way to go, so no hard floors, and you might very well need treatment at ceiling level to stop first/second reflections there. The smaller speakers that give more flexibility to position mean less room treatment will be necessary. If it's also a lounge or used for other purposes, then this stuff isn't exactly desirable to have in the room!
Sorry it's not the answer you're look for, but it's going to be
the message in 2019: The biggest influence on sound in a room is the room itself.