Pass on the RELs as they do not have sufficient controls to integrate properly. IMO REL now markets bling as opposed to decades back when they were excellent.
Required controls are continuously variable XO frequency, variable XO slopes, continuous phase and polarity inversion. Without these controls only physical movement of the sub enables proper integration. Martin-Logan X series have the requisite controls.
It’s simple to build a HP xover for the mains which enables proper integration. Rolling the bottom octave out of the mains effectively increases main amplifier power.
Subs have significant output into the mids and are localizable. Therefore they must be coherent with the mains. The easiest way to achieve this is to have them in the same area as the mains.
See ieLogical SubterraneanHomesickBlues for more information on sub integration