Not sure which REL sub you have and what kind of connection/plug is used for that. On mine, there is a big blue plug and getting that would be my primary question.
Re AWG size, as the signal is low level (= about the same as interconnects) don't worry about AWG as you might with speaker cables. Even then, I have not heard any difference between AWG 14 and 4-5, which are MASSIVELY different cable diameters.
I would just use the REL cable and enjoy your music. Placement of sub and setting cross-over frequency and phase have MUCH greater effect on sound than any cable by several orders of magnitude. That is where I would focus my attention.