IMHO, the key to successfully integrating any subwoofer, is to use it to subtly augment just the lowest octave of the bass you already have, and not feature it. Assuming you want natural musical bass, and not dance floor thump, I’d run main speakers full range, feed the high level input of the sub with the amp’s output, set the low pass filter to it’s lowest setting (40-50hz), and set the gain just loud enough so you can barely detect that the sub is on. The REL and your Willsenton with your speakers should work really nicely once placed well and dialed in suitably. There’ll be plenty of trial and error to optimize it, but that’s part of the fun.