Why not use the Delta 15, and combine it with a I PI, all in a single box. This could be configured somewhat like the old JBL 4698b Cabaret but with a lower cross-over. I was going to suggest the Omega 18 but the efficiency mismatch is 2db which would mess things up. If the cross-over from the 15 to the 8 was about 300 hz then passive components would not need to be very large or expensive, the sensitivity would match, and yet the midrange would not be compromised. At the same time, I would think midrange power handling would be sufficient for almost any kind of music. The piezo tweeter obviously fills in the top end. Seems like the 115 db you mentioned in a previous thread for the piezo out to the top end would be more than sufficient for this limited frequency range. Wouldn't quite match a Cornwall for efficiency but ought to be flatter, especially on top.

Thoughts?