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%22

I 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.

6000 watts continuous call me over when your ready to plug it in please.

I have two of the EVX-180B in my home theater system. I have each one in a 18" Super Scooper. They are being pushed by a Phonic 3200 amp, running in parallel. So I'm push 2800 watts to them. Playing rock music thru them and I knock pictures off the wall in my living room and can hear them at my next door neighbors house hitting lowes.

The speaker is great and I have always wanted to build a HWK-18 for them but never have. Let me know what they sound like.

Good luck in building the HWK-18.