It takes a full range bass signal, crosses it over so only freq's of 120hz or so get through (it provides a separate output for 120+ to go to your mains), does some pretty intense processing and finally kicks out a signal that lets a sealed cabinet produce ultralow frequency output (as low as 8hz). Bag End builds cabs it now calls INFRA (new name, exact same as ELF) to work with this integrator). You put it in between a preamp and a dedicated power amp or dedicated channel from a power amp (can't daisy chain an ELF cab with a traditional cab) and it will produce bass or PA sound with incredible fidelity to lows you've never heard outside of very high end arena sound systems. To make us of it you need a sealed cabinet (see Bag End website and look at the S18E-D for an example).
Without a sealed cab, the integrator is useless. With the right cab (Bag End seems to be the biggest maker) it is very impressive. I've used an ELF integrator (it consumes one space in my rack) with a Bag End S18E- C for a couple of years as a sub to go along with a 212 and sometimes a good 115 (with horn). My sound constantly draws compliments and amazed looks from other bass players. More recently, I bought a second ELF 18 cab to use (with the integrator) for PA. The ELF pair did a creditable job doing subwoofer duty at an outdoor gig on a football field and kicked ass in a fairly large club indoors. They are on the esoteric side for bass rigs and small PAs, but they absolutely kick butt if you appreciate great sound quality.
