I've used dual horizontal MTM centers for a long time -- MTMMTM -- and although I take a lot of cr@p whenever I post this, it works really well.
As many who want to teach me Loudspeakers 101 so accurately point out, long arrays of speakers tend to 'beam' -- the center lobe of the diffration pattern gets narrower, and the usual +/- 20 degree off axis seating recommendation that accompanies most horizontal MTM's shrinks.
But audio is all about compromises, and although you can notice differences in FR outside the center lobe more than with a single MTM by paying attention as you move from one seat to another, these differences are overwhelmed by the overall improvement in SQ provided by more radiating area and reduction in required excursion for a given SPL.
And once you start improving on the basic setup -- both speakers facing straight ahead and together -- and start angling then and spacing them to best cover your seating positions, you can almost always do better than a single unit (IMO).
This setup works well with 6.x" or smaller MTs, MTMs, or TMM's.
If you have the space, identical full range speakers in all positions is the way to go -- full range implying the ability to generate concert levels down to 40-50 Hz without distorting higher frequencies. If you don't have space for vertical designs, horizontal W T/M W designs do have an advantage in horizontal dispersion and distortion, but you have to pay for it.
And of course there are coaxial and full range driver systems.
If you have an acoustically designed room and have a large budget, you can do better (obviously). If you are after pin-point imaging any horizontal array is questionable. But for those of us who find the price/performance of quality two-way systems optimum and who have typical TV setups requring horizontal speakers, doubling the center can and does work.
As an added bonus, you have the thing that makes this work best -- an additional amp channel so you can power each speaker individually.
And -- I've NEVER heard an above-the-screen / below-the-screen setup work. Not only are the centers too far apart to integrate, but the different refection patterns from the floor and ceiling keep the sources "psychoacoustically distinct".
Finally to address the question you actually asked :), different voicing for the VP150 vs the M60s:
IF the difference you hear is in the lower midrange and bass, two ported M22's will get you closer to the 6.x" woofer in the M60s, but for best results you'll need banded or parametric EQ on the center channel -- and you are still going to need to cross to a sub by 80Hz.
Things you can try that don't cost any money:
Run you sub xo up to 100 Hz either on the VP150 alone or globally.
Play with any center channel EQ you have.
