Before you do anything physically exhausting or start spending lots of money try setting the speakers to wall and if that doesn't help corner. I had huge bass problems with DSP5200s and setting them to wall fixed them right up. If I remember correctly the recommendation is something like within 50cm of a boundary to change them from free so certainly the back wall isn't helping. I think if you power them on whilst pressing 0 on the remote you should get into the settings where you can change the speaker position.

Anthony.

Sources: MS600, Fidelity Audio Reference Oppo UHD-203, Roon/QNAP, AppleTV 4K
Processors: Arcam AV40, 271 x2, Lumagen 5348 with Keces P8
Speakers: DSP7200SE FL/FR, DSP5200SE VC, DSP3200 x11, DSP320.2 VOG, DSW x4
Display: JVC DLA-N7 with PK-EM2, Panamorph Phoenix Pro, 128" 2.37 Curved Silver Ticket AT Screen