REL subs are superb, IME. Having said that, to add just one to a pair of monoblocks is not ideal. Since REL subs are balanced by design, a single sub running off the taps of the amps ( not speaker) can be problematical. REL suggest two (2) subs if a mono block system is utilized.That is how I have done it with my mono block amps.
How to connect a subwoofer into a mono block system
I'm interested in adding a subwoofer into my audio system. I have a new Audible Illusions M3B driving Welborne Labs Laurel Iix ultimate mono block amps and ProAc 50 Signature speakers. I've been confused by material on the web as to how to do this so as to insure that the subwoofer is seeing exactly the same signal that the ProAcs receive. Any recommendations, both as to how to best do this, but also on subwoofer choices?