Simple, RCA out from pre-amp into the sub, then adjust the sub’s crossover, volume, and phase. Finding the best crossover typically requires an SPL meter, but it can be done by ear. Finding the correct phase is easy to, but if you want me to detail it, I can. Finding the correct volume is also not difficult.
Make sure to do the sub crawl to find the best position for the sub (if crossing over >60Hz, find the best spot close to the speakers.
Make sure to do the sub crawl to find the best position for the sub (if crossing over >60Hz, find the best spot close to the speakers.