For MBL, and I would assume all omnis, SMT wings is the best solution I have heard. It is used in most shows by MBL North America team to great success. Their rooms are packed all the time and they consistently win "best of" in multiple show write-ups by the "pros" and fans alike.
For MBL, SMT recommends their treatments directly behind the speakers, several panels starting on the sides of the speakers (preferably up to the listening position, but can stop a few feet earlier if funds don't allow), and a small wall of 2-3 wings/ 48-72 inches about 2 feet behind the listeners head, and, of course, their helmholtz diffusers if funds allow.