Mixed reviews on the above product.
americanaudiophile: No I didn't assume that. I ASSumed, based on the info provided: that there's a subwoofer with a line-level input, which means it has it's own amp. I also assumed, because he mentioned the "SINGLE RCA INPUT" and not the SUB output on his amp, that he needs to also get a MONO signal (again-single RCA) into the amp (I didn't address that) and that he also needs to get a MONO SUB signal from the amp's SUB OUT to the SUB's input. That's what I addressed.
I suggested getting decent quality SHEILDED cable and the link I provided is 3-conductor (L+R+sheild) to get a line-level MONO signal to a single RCA (which is MONO) input. Didn't he say he has a "single RCA input" on the amp" and that it's "line" level?? Going by the info we're given here, the existing 12/2 would be a bad choice for LINE LEVEL. The best way for him to get CLEAN, MONO SUB line-level signal is to do what I suggested with a shielded cable, considering some lack of info here (like the sub).
aliceinca, You know what they say...two (or more!) heads are better than one. Consider your options and do some quick research: you may want to call SONOS, call your sub mfr. I've been doing stuff like this for decades and although the tech has grown by leaps and bounds, you still need to accomplish one thing: to get the signal from point A to point B. How you do this is up to you. From a high-end audio point of view, I'd choose the option of getting the cleanest signal to the amp. 40 feet is a long run. I've seen signal degradation from various wifi and blue-tooth rigs, so I opt for the direct connection whenever possible.