I really don't want to spend a lot of money on a cable. What would be an inexpensive way to buy or make a 50ft cable for me REL subwoofer? Would it be ok to just attach an extension to my existing cable?
Yes that would be fine. I really don't think you would have a reduction in bass, I would just splice some unto your existing Rel sub cable. Just make sure the bare wire is wrapped in electric tape to ward off any shorting.