I'm presently using my pair of 34 lb GK 212MBE 2x12 cabs powered at 600wrms each as subs when I don't need more. They are the 4 ohm versions and powered with a Behringer NU3000. The drivers 8 ohm neo 300wrms GK.
I used to do something similar. I had a pair of JBL JRX-125 mains (which for what I was doing sounded great on their own), and for larger rooms with raised stages, or outdoors, I added a pair of Avatar 212 bass cabs, each about 1/3 in from the sides of the stage, tucked along the front of the stage to use the floor and stage knee wall as a great big folded bass "horn." People loved it, and it sounded great. I was mixing almost all kinds of music, but mostly classic and alt rock. No mud, just thump.
I think the JBLs would go down to about 35Hz on their own, and that's about where the Avatars bottomed out, too. The JBLs were by no means flat that low, and the Avatars really helped to even out the range without really pushing amps too hard.
My current rig is a bit more modest -- a pair of 15+horn cabinets on sticks (Peavey PVX15) and a Mackie 18" powered sub. It rocks well enough, though the mains can sound a little boxy in some rooms if I'm not careful with the main EQ. The next audio project is to build a pair of 15" psuedo subs. I've designed the L-slot-ported boxes with a resonant frequency of about 30Hz, and the drivers I'll be using start their rolloff at about 40Hz. In theory, they should be pretty solid in the 40-100Hz range, and I'm expecting to cross over right around 100Hz or so.
I cart my PA gear around in a Hyundai Elantra Touring (i30), along with my keyboard rig. It's a tight squeeze, but I get it there!