I love the idea of having two separate systems, one for music and one for HT. That said, I am having a hard time overcoming the amount of equipment clutter this will create ... 5 speakers, more subs, receiver, interconnects, cables. I guess that might just be a “me” problem.
I already have 2 REL 812/S subs. Adding two more is something I was considering just for music, nice that it makes sense for HT as well. REL is very clear that their subs should be connected to the high pass on the sub and from there directly to the amp for music … no connection to the preamp. This is the way I have set them up. I guess/assume that for HT, the connection to the subs should be to the .1 LFE and then to a receiver(?).