So, having finally decided that a couple of imperials would take up way too much space, I have decided to build a HWK-18 using two of these EVX-180B drivers:
http://www.cameronproaudio.com/pdf/evx180b.pdf#search=%22evx-180b%22I haven't been happy with the way my HWK-15 was performing (2 Dayton Series II drivers, both magnets in center), ever since I tested it with the two drivers clamshelled in the bottom baffle. The response in that case was much lower, great for HT/LFE but peaking a bit too low for music. With the drivers as they are now, bass is tight and reasonably musical but the extension is not as low. I'm hoping that the HWK-18 with the EV drivers will cover most of the musical requirements, allowing me to switch the HWK-15 back to the clamshell-in-lower-baffle configuration just to cover the bottom octave. With an Fs of 19Hz, the Daytons are pretty well suited to that purpose.