Speakon connectors are very easy to work with. They have set screw type internal terminations and come with two sizes of inserts to use as strain relief. I just made two different umbilical DC power cables with Speakon connectors to connect an outboard power supply to a preamp. For one cable I used bulk power cable and for the other I used bulk IC cable. These are terminated with Speakons at both ends.
I assume the REL comes with a Speakon input? If you know what cable you want to use, you could easily reterminate it with a Speakon connector, which is easily (and inexpensively) available from Parts Express. If you want to try a variety of different cables with your new sub, you could consider wiring up a Speakon connector to some short hook-up wires and then terminate those to a standard rca connector input (I assume you are connecting a standard low level sub cable?). An inexpensive rca input connector should also be available from Parts Express. Using that "adaptor" set-up, you could try a variety of sub cables that have standard terminations until you zero in on what you want, and then reterminate the REL end of that cable directly to a Speakon connector. I would try something like that before paying a bunch of money to have someone make a cable that I haven't heard before, or to pay to have someone reterminate cables for me.
