I'm self-doubting whether to do a better 2x4" MEH or (using 16ohm 4FE35) 4x4" square MEH. With my "more coherent" 1st-order-notched T/MM I think I heard a bit of HF108 ketone polymer membrane sound signature, trading steely edged strings for hard plastickyness -- a little bit. My Minmeh 2nd-order-notched, flatter FR didn't do that.
Please consider giving "reflector point-source" topology a test. Just point a small car tweeter at midwoofer dustcap and move it around until their acoustic centers (one real, one reflection) became coincident and the sound holographic. Also LX which reduced the acoustic centers gap beween midtweeter and up-firing midwoofer.
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Omnidirectional has it's use and users, but it can be hard to find a proper placement, taking up to much space (that could be used for a BIG horn 🙂. Depending on what ones intended use is, probably somewhere between critical listening and background noise, one should aim for a suitable system. I would prefer something that is true to the recording, so I would try to avoid early reflections (<10ms) to "hide" the room. My idea for a horn is to utilize the ability to (somewhat) control the beamwidht, and getting single-point source would be a big benefit. If in need of more reflections, one could toe the speakers in/out, but that would demand speakers suited for the listening room and/or vice versa. DIY can make this happen 🙂
Sure, depends on the room, placement, and seating. Many ways to reduce dispersion & reflections; not so many to get point-source (as you've said). LX cardioid-dipole can be close to walls. Point-source reflector size/shape is diy to taste (midwoofer face front or up). I'm doing MEH in a smallish room only 3.5x6m so directivity helps. Not really sure 8cm vs 12cm effective CtoC would make much difference far-field in a large space.
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^ Agree. CtC-distances can be somewhat relaxed with correct filtering, details at Linkwitzlab. My listening room is somewhat like Yours, +low ceiling LWH 717x336x219cm, so for me a 64x46 degree horn would be nice, but it has to be BIG to get that pattern and have loading down to the Schroeder-frequency. One not-so-obvious possibility with MEH is the low group delay at low (bass) frequencies. Any abrupt changes in a horn will give reflections, reflections will give a ragged impedance (and freq) response, the same seems to apply for the mouth => a rounded mouth should be beneficial.
Sketch of predicted in-room reflexes. Dots are ego-centric listening-positions 😎
Sketch of predicted in-room reflexes. Dots are ego-centric listening-positions 😎