Sometimes, I've used them with a receiver with a dedicated subwoofer output with adjustable crossover frequency, along with a separate power amp. I've even used a car stereo electric crossover powered by a 12 volt power supply that would plug in the wall, along with a separate power amp.
I think the next thing I will try will be a crossover similar to the stock one, just with a lower crossover point. I got a couple of these on e-bay from someone selling surplus CerwinVega parts a few years ago for a project that never materialized. In this scenario a separate amp won't be necessary.
Most of my tinkering is free entertainment, because I use stuff I already have lying around. It might not be financially efficient to copy any of my efforts if you don't already have the stuff lying around unused.
Probably the cheapest route would be a passive crossover like the one I plan on trying when I get around to it. I just looked, and e-bay seller "mavin" is selling some crossovers similar to what I have and at a cheap price, too. Any of the other methods would involve spending more than you would on a powered sub from Best Buy.