The closer the OB is to the front wall, the higher the cancellation that between front and back wave, and the shorter the time difference in arrival times between the direct and reflected sound (per Ralph's post above). So further away from the way is better for both how low end reach and the sense of envelopment.
Linkwitz recommended a reflective front wall, which happened to be what he had at home. I happened to have a treated room with absorbent front wall and can get my speakers 3 feet away from the front wall, and I'm happy with the sound. Maybe I would be happier with a reflective wall? Dunno. The midbass, as noted, lacks some impact even though it's 18", have a dedicated Hypex class-D directly powering it and it's adjusted to measure flat...yet the sense of impact is different in my case.