It's not that expensive, man. The least expensive ELF integrator, the M, is the one to use for bass guitar. The response is flat (flat, Bruce, flat!)
to 18Hz. They cost about $800, and you can get a used one for $600.
Only the higher-priced models do the 8 Hz thing. The more you pay, the more features you get. Any of the 3 will work, but the M is designed for musicians in particular.
The cabs are not any more expensive than the regular Bag Ends. BassNW has had some used ones in the past, ranging from $400 for a 1x18 to $450 or $500 for the 2x12. So for a grand you can have a sub that is FLAT to 18Hz. That means it is also flat to 31Hz.
ELFs are also louder than conventional subs, and don't roll off at all. They come in 1x10, 2x10, 1x12, 2x12, 1x18, and 2x18. Sorry, no 15's.
And the biggest advantage is the small size.
As for the signal chain, you run your signal into the ELF rack unit, and the output of the ELF goes to the power amp. You can feed your other amp/preamp from the back of the ELF, or vice versa. ELF doesn't go into the effects loop, tho.
Of course, conventional subs can be cheaper, but what models exist that are the same size as a regular bass cab, weigh no more than 100 lbs., and are -3dB to 30 Hz? Can anyone think of any?