How to Add a Sub on a 2 Channel System?

REL powered subs recommend using the high level input for music. There is a cable supplied with the sub that connects to the speaker binding posts on your amp.

Rooze, I think it depends on the design of your amps, but REL recommends two subs with most bridged mono setups connected from the speaker terminals on the amp to the high level inputs on the sub, one sub on each amp. I'm assuming you have one amp on each speaker now. In that case, no need for a separate active XO and it would not go through the maggie xo either. You would still send the full range signal to the speakers and the subs would be set to fill in the lower frequencies where the mains roll off.

You may need to contact the amp manufacturer to get their recommendations, or someone here may know if you provide more specific information.

The REL sub I have is the Stadium III. It came with a high level, 30' cable that has three wires and a Neutrik Speakon connector for the sub. With my single amp, I connect a wire to each channel on the + terminals and the remaining wire to the right channel - terminal. The Speakon connector goes to the high level unbalanced input on the sub. With bridged monos, the REL manual says to connect one sub to each amp on the + & - terminals and the third to the amp ground. The Speakon connects to the high level balanced input on each sub. That would help keep the cost down.

You should ask Sumiko , the REL US distributor, for suggestions on running a single sub with mono amps.
You do have the option of using the line inputs on a single sub, but REL stresses the benefits of the high level connections so I would try to do that first.

Some people use two Strata subs and report excellent results.