spoutjack's original question made it unclear as to whether he was asking for a high level/speaker cable recommendation, or a line level/RCA jack cable one for use with a Rythmik sub. My question was intended to get clarification because Rythmik subs are available with a choice of two basic amp groups: 1- the PEQ version, with both RCA jacks/line level inputs AND binding posts/high level inputs, or 2- the XLR version, with both RCA jacks/line level inputs AND XLR/balanced line level inputs but no binding posts.
Brian Ding of Rythmik recommends using the RCA or XLR jacks/line level inputs in preference to the binding posts/high level inputs whenever possible. Other sub makers and users, particularly REL I believe, recommend and prefer speaker level hook ups. The Rythmik PEQ amp allows both, the Rythmik XLR only line level, not speaker.
For hook up to a Rythmik via speaker cables, one is free of considerations other than bass/low frequencies, so a regular speaker cable known for it's good bass reproduction is an obvious candidate. Some cable companies make cables specifically for subs, but I've never tried one, as all my Rythmik subs contain the XLR version amp (370w and 600w).