It's a bit tallish, but using the existing unmodified drivers(the tweeter apparently is the one with the QS4 face plate)limits the extent to which the height could be minimized. It would be nice if it could be lowered an inch or so, but this would require modifying the tweeter and 5.25" face plates to fit more closely together.

Enclosure size apart, however, this promises to be an excellent sounding center when development is complete. I'd guess that a likely price would be around $540.

Ian, if you're of a mind to do a little more experimentation in this area, one of the thoughts that I've mentioned here in the past was a smaller W T/M W using two 5.25" woofers, the QS4 form of tweeter faceplate and the 4" mid-range from the QS4.
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Enjoy the music, not the equipment.