If you have a mono sub out, there's no need or advantage to using a Y to connect both L and R inputs, as far as I'm aware. The only reason to connect both the L and R would be is if you're connecting a device that doesn't have a dedicated sub out and you're taking the L and R full-bandwidth signals and using the sub's crossover to "extract" the bass.
Your "speaker sub out" link is broken, btw.