You're saying that Rythmik says to use that Mediabridge cable? Brian (Rythmik) is an Electronic and Acoustic Engineer out of USC. I would absolutely trust his opinion on anything HT related and in particular Subwoofer related. (I have visited Brian/Rythmik in person and bought a F15HP).
I've been happy with the quality of the Mediabridge stuff.
Just 1 item to mention, from what I've seen on Mediabridge, their RCAs terminals tend to be a little longer/bulkier than average, so if you don't have plenty of room between the sub amp and the wall, it could be a problem. Nothing a couple of 90 degree RCA terminals won't fix.