B_limo,
With my OHM omni's, speaker distance apart has little effect on soundstage width. It is wall to wall, about 20" total even if speakers are just a few feet apart. My room is L shaped with the OHM's sitting at the base of the L firing into the long dimension, which is narrow. Omnis are are a perfect fit there. I had Maggies there prior and could never get them to work as well as I knew they could from prior experience with them in other rooms.
Achieving soundstage depth is the biggest challenge in this room as with most in that having speakers too far out from rear wall is a practical problem in many cases, but the OHMs do a good job even with just enough distance from rear wall to avoid early reflections. I like the distance the reflected sound travels to be about 2X that of the direct, to the extent possible, for best results.
A large room, wide and deep, with lots of space behind, does help not only with enabling a large soundstage, but also helps our ears to be able to triangulate on sound locations within the soundstage better and with more detail, ie perceive imaging within the soudstage accurately. Our 2 ears and two eyes work similarly in this way in terms of being able to see or hear in "stereo" and being able to locate things spatially in three dimensions.
Generally, you will want more room behind the speakers to work well with more space between. WHen speakers move closer to rear wall, less space may be best between speakers as well. It's mostly about getting the right distribution, balance, and magnitude of direct and reflected sound from the two stereo speakers at the listening location or locations.
An excellent tweak otherwise would be to move one's ears further apart somehow. Not likely to happen though, so lots of open 3-D space for the speakers to image in is your friend in general otherwise.
Things work similarly in smaller rooms, but on a smaller scale. Its like having a 15" TV versus a 70" big screen in essence. If set up right including listening position, similar good things can still happen, just on a smaller scale.
I find omni's overall to be easier to setup for good overall results than more directional designs, though fine tuning for absolute best results is still needed as well.
At my reference mbl demo behavior appeared similar ie distance to rear wall was the key as long as early reflections from side wall were not in play.
That room was also heavily treated with curtains and other soft absorbent materials, which were absent at show demos with lesser results. Also the room area behind the mbls was tapered, not rectangu;lar, much like a musical instrument, probably to minimize resonances.
If I had a room like that, I think there is a chance my OHMs could probably challenge the mbls for soundstage depth and maybe even beat them in most every other way, but I may never know for sure.
My big OHMs ended up costing me about $2500 a couple years back, with sale price at the time and maximum trade-in value (40%) applied. They go for about $6500 brand spanking new these days, less if OHM Walsh CLS driver "can" is mounted in older refurbed cabinets, like mine, which are refurbed OHM F cabinets that could be a good 30-40 years old by now.
Dynamics are top notch as well. I do not feel wanting after a good horn speaker demo when I come home.
The big mbl 101s may be the boss in terms of bass output potential and dynamics there though. THe mbls I heard were "only" 111e's. Have never had the pleasure of hearing 101s.
TO challenge bass output from 101s perhaps, OHM now has a 5015 model with powered subs integrated into the cabinets for about $10K. I would love to hear a shootout between those and mbl 101s each set up optimally.