Try playing a test tone at approximately the crossover frequency. Whichever gives the loudest output is the correct setting.
On my Denon AVC-A1SE with Velo HGS 15, the correct setting was with the phase set to 180deg. I have no idea wether this is because the Denon reverses phase or the Sub does, it doesn't really matter. One of them does and that is why the switch is provided.