I have a dedicated room the same size and I added 12 GIK 242 panels (4 front wall, 4 back wall and a pair each side wall) which made a nice difference. I emailed with them a few times and sent them pics and REW measurements. For my budget they recommended their 242 panel so that's what I got.
I have a pair of Aurum Cantus V7F speakers and when I play it loud then yeah it starts to overload the room a bit but I don't play them that loud ever. I tried putting the system on the long wall first then tried the short wall and that's the way it stayed.