Here's another reply from one of the big-dogs of the Motif Forum. Someone who has even put their hands on the MM6 too.
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Having spent quite a lot of time playing the MM6 I have to report that the polyphony is a bit of mystery. 32-voice would seem to be on the light side. However, despite playing layerered sounds using clustered chords on top of fairly dense Performance backings, nothing chokes and it all sounds great.
To echo YamahaUS's comment about this being a different and new type of animal. It really is. It has the sonic heft of a Motif but with instant gratification features of beats, arps, and (pretty darned sophisticated) chord recognition. It's one of those instruments you just can't help but 'keep on playing.'
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GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.