+1 - I agree that MTM speakers on their side are not ideal for center channel speakers - Def Tech isn't doing anything about it but other speaker manufacturers are like Paradigm, NHT, SVS, etc with a proper three way Woofer-Tweeter over Mid-Woofer design and a very low crossover frequency to the separated woofers.
My 15 year old set of three identical Paradigm Reference LCR-450a (MTM / D'Appolito design) speakers sounded great on their ends, but the one in the middle on its side always sounded so different (even though it was an identical speaker). As soon as I upgraded to the Paradigm Studio CC-590 (WT/MW) things sounded incredible.
One way you can rule out the design of the center - sit exactly in front of it. The separated mid-range speakers will work as one at all frequencies in exactly one spot - the dead center. If you still have intelligibility / clarity issues when you are in the "sweet spot" there could be other issues like interference from the cabinet.
Is there no way that it can go in the cabinet right on top of the TV?
2-Ch (HT L/R): Oppo BDP-105 BD, Adcom GFP-750 pre, Bryston 10B Sub Xover, Bryston 4BSST2 / Paradigm Signature S4 v.2 (L/R), (2) SVS SB12-NSD (Subs)
Home Theater: Bryston 4BSST2 amp / Paradigm CC-590 (C), Outlaw 7700 amp / (4) Def Tech UIW-RSSII (LS/RS/LB/RB), Samsung 46” 3D LCD