sub phase switch

[ton]

what is the purpose of this and what is the recommended setting?

[BradJudy]

If you are familiar with a frequency response graph, this should give you a good idea of what the effect of a phase switch - http://www.bradjudy.com/audioblog/2005/08/30/the-a-difference-a-phase-makes/

In short, it helps optimize the integration of the sub with the speakers and should be put on the setting that makes things sound a bit louder.