It looks like a great setup but there are a few things you might consider before going to a subwoofer. It seems to me that both imaging setup and bass setup is critical:
1. Repositioning the speakers. They look too close to the side walls and aimed straight ahead. Consider moving them and toeing them in to cross in front of the listening position. This way you will be as much off-axis as now but soundstaging should be equally good but positioning less critical.
2. Consider other/different room treatments. The effects you are experiencing may be due to an excess of mid- to- high frequency absorption but little effect on the residual room modes. All that relatively thin foam, even the stuff in the corners, will have much, much less effect on the bass than high density fiberglass bass traps. You can do this best with decent bass traps (RealTraps, GIK, etc.) but another option is the Rives PARC which will do this electronically.