How to Add a Sub on a 2 Channel System?

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.

Thanks again Eagle

I am not a huge REL fan, although I believe they make wonderful equipment. With your current set up, and quest for a "musical" sub, the Vandersteen 2wq is the only way to go. Since it costs much less than a REL, and since you have such wonderful speakers, 2 would sound fantastic is the room if large enough.