imhififan,
Looking at the Jensen unit it looks like you have to use speakon connectors, is that true?
Update:
Just talked to Parts Express they had no idea how this thing works, no help!
Probably the easiest and best way would be to use either an electronic crossover, or just simply use a y adapter. One end goes to the main amp then speakers, the other goes to sub. You can adjust the sub for the proper low level point, however your main speakers would be playing full range. There may be a hump where the frequencies overlap but with adjusting the crossover point on the JL it can be minimized.
ozzy