A few comments:
Yes, move the seat forward. You’re probably hearing a lot from the rear wall you could eliminate by moving forward. My speakers are 8 feet apart and I sit 10 feet away. My back wall is another 9 feet behind me. Worth a try.
Did I miss comments about ceiling height? Some of the worst reflections come from floor or ceiling. They may be interfering. It's very helpful to measure impulse responses to see how the reflections are, or you won’t get a handle on imaging and soundstage.
Get those as many reflections between 4 and 12 ms down below -20db if you can. Viz.
- There is a roll-off before 20kHz.
- A dip is present around 12kHz.
COMMENT: These are very high and likely won’t matter much. (In my view.)
- There is a noticeable boost between 50Hz and 1.5kHz.
COMMENT: Much more important.