High level speaker amp out to RCA

Hello community.

Many preach, including REL,  that the best way to connect a subwoofer is from the high level speaker output from the amplifiers so the sub receives the same "signature" sound the amp sends to the speakers. This sounds quite logical!! My subs only have RCA inputs, how can I go about this type of connection? Is there a good high level to RCA thingamajig?? Any suggestions??

Thank you for your constant guidance!!