Kclone: RELS are unusual in that they recommend binding the sub inputs to the binding post of the speakers.
Therefore, I don't think they introduce any crossover into the circuit.
It may be possible to wire them in different ways. Look at picture of backpanel and see if there is crossover.
Personally, I would get two subs and highpass them in series to my mains, then set up the room carefully. I would get sealed subs if I could afford them.