The cubes' RCA connectors were used to more conveniently connect to the bass module, which contained a crossover network. You should be able to snip the RCA's off and strip the wires (or get some RCA extension cables and snip an end off those to preserve the resale value of the cubes), but there's probably no obvious indication of positive and negative so polarity could be an issue. If the wires have some sort of mark (ridges, stripe, text, ...) on one conductor, that would allow you to be consistent across all channels.