http://www.dbxpro.com/product_downlo...s%20Manual.pdf

This is basically an electronic crossover, but with some interesting bass options.
You could set it up ether way, just use the MR918 to supplement the bass of the SR4735 or go full bi-amp.
I would favor the full bi-amp mode, by reliving the SR4735 of that really low bass the mids that are coming from it will be cleaner by reducing the inter-modulation distortion and this will lessen the power demands on the JBL amp.
Keep in mind the that for every octave down you go the cone excursion doubles. If the cone has to move 1/8" at 200hz it will have to move 1/4" at 100hz, etc.
MR918 is designed for that low bass, run the system full bi-amp and let the MR918 do it's job.
Also read the manual and follow the direction on using the bass synthesizer part, proceed slowly with caution, on older recordings where the bass was limited you can restore most of it, but be careful!