I went through the exact same situation with my REL. REL service is no help, unfortunately. My solution, that I am quite happy with and with sound quality that is as good or better: The REL plate amplifier is easily removed and the box remains sealed allowing easy access to the connection to the woofer for use of:
https://www.parts-express.com/dayton-audio-sa230-230w-subwoofer-amplifier--300-813
https://www.parts-express.com/dayton-audio-sa230-230w-subwoofer-amplifier--300-813