I use to run 2 REL stadium's in stereo from speaker terminals as per REL's recommendations.
But now have 2 Fathom 12's running off Nad master series preamp which is a mono outlet. I'm using on board crossover in preamp at 60Hz. Movies are fine but music does worry me. I would have to split bass off from main preamp out which is not ideal. The Good sound doctor Miami says what I'm doing is fine. I felt there was a sound improvement going to sub out and using on board crossover. My main speakers seem cleaner more dynamic and smoother by removing low bass. Fathoms are incredible so still a bit overwhelmed by there presence.