The smallest sub I ever heard that actually sounded any good was the EV sb120a. It was only a 12", and it was the same size as the SX series cabinets, but the sound was great. Not THUNDERous, but nice and deep. I think it's discontinued, probably because the 15" and 18" are so much more efficient in the low range. Most people abuse speakers and if you pushed the sb120's they would start to splat. Too much splat and you get .................. the sound of silence.

If you really need the bottom end and don't want a dedicated, large sub... try the Peavey Kosmos with any 15" bass amp. Get a 2 or 3 hundred watt single unit combo amp.....plug the Kosmos into the EV's and send the sub out to the bass amp and you'llbe amazed at the difference. The only other option is to carry something large and bulky. Bass requires air space, both in front and INSIDE the cabinet. It's physics - NOT ergononics that counts when moving air.