I want to build something like this

[Fast1one 10+ year member]

I ahve a "real" sub as wellll.
WOuld I still use a 15 or 18 for midbass/

These two drivers have been used by a few people over at diyaudio with great results:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=290-407

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=292-218

Also this 18 has been used by Martin J. King over at quarter-wave.com with great results:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=290-386

Since you already have a dedicated subwoofer, you wouldn't have to worry about EQ. I would follow Martin's guidelines over on quarter-wave for the proper crossover points and slopes.

I would just go to your nearest Home theatre store and pull out the CC and buy something...

Umm...no
Well, I suppose it depends which home theater store. And who you know //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif