I have a Bel Canto C5I integrated amplifier with a line out going to a REF500S a single dual mono amplifier. I use the high output of my Rel TX8 as follows. Red lead into the Right positive ' and Yellow into the Left positive. DO NOT CONNECT THE BLACK WIRE !! tape the end and leave it hanging. You will need to do this with "each" of your monoblocks out to both of your subs as you are using 2x.
The Black wire is connected to a negative OUTPUT on Class A or A/B AMPS. But not on class D as in the case of this particular bel Canto Amp series there is the voltage of around 15% on the negative terminal on class D amplification. You can connect the black wire to class A or A/B as there is 0 voltage. If you introduce voltage to the REL it could damage the amp