Andrew, as Randy comments, the configuration that you describe would most closely follow the VP180 pattern, with tweeters(of course a second tweeter would have to be added)inside the woofers and mid-range(s)in the middle. It wouldn't appear to be necessary to use a larger mid-range, since the M60 of course uses the single 5 1/4" mid-range now.

In line with the comment by Dr. House, over the years I've mentioned several times the possibility of using a W T/M W configuration(possibly using the 4" mid-range from the QS4s). Although the same drivers(no additional tweeter)could be used, unfortunately there wouldn't appear to be enough space on the M60 enclosure for the 5 1/4" midrange and the tweeter to be positioned vertically, even if the tweeter surround was partially cut away to allow closer spacing.

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Enjoy the music, not the equipment.