The more subs the better, but I agree, with speakers like those, tread lightly - only use very good subs.
I don't have speakers of that caliber. But I do have 5 RELS in a huge carpeted bonus room (entire footprint of a big house) and it took all those subs to make the room.
At this point, I truly marvel at the tightness of the bass. And BTW in terms of dialing them in, the gain on the subs needs to be truly low - can't hear any individual subwoofer. If you can directionally hear it, not dialed in.