I have two REL Classic 98 subs, they definitely sound best using RELS "high level input" cables hooked up to the speaker terminals on the amp. The "low level" RCA inputs are a bit muddy sounding IMO. I would not use the Parasound crossovers in the amp, the REL subs already have built in amps/crossovers and you are just making things way too complicated trying to use their High and low pass features, just use the "High level Inputs" on the REL.
Pay very close attention to the way you hook the wires up to the speaker terminals. Since my amps are differential mono blocks I had to do things differently to solve a 60hz hum.
Your Hint 6 should (???) hook up this way:
Connections should be made to the same binding post on main amplifier as the main speakers. Red (+) to
amplifier main right speaker red terminal (+), yellow to amplifier main left speaker red (+) terminal and black to
a ground lug or chassis screw.