Think subs you'll be fine. I split the signal out of the preamp. One to the active XO and one to the other speaker amps. I use Behringer, First Watt, and two valve active crossovers. They are all a little different. BUT I only have two active XO that are good enough for mids and HF drivers. They are are made by STL and First Watt. I made a good HF Active.. It wasn't to hard..
The 1/4", RCA, XLR or anything else should work plugging into the REL with an adaptor. What you do have to be careful with is wiring a REL to a Class D speaker terminals, there is a proper way to do it. REL says it will harm their amp. Not yours.
I'm not a fan of using speaker cables to power subs. There is to many great ways other than that and the convoluted wiring crap REL folks have to go through. I REALY don't know why. LOL As you can tell I'm not a REAL fan..
Nice paint though..
BTW I have a DIY repair for the decoupling issue.. 50.00 or so.. Or a gallon bucket of super glue that works too.