Talk to the folks at GIK Acoustics. They'll advise over the phone. Their wall mounted 244 bass trap panels behind speakers and 242 panels on side walls and on wall behind listening position made a big difference in my room (similar dimensions as yours but more reflective, I'd say). Floor standing soffit bass traps in corners have much less of a noticeable effect though I continue to use them. 244 & 242 panels definitely some of the better money I've spent on audio, in my opinion.
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