The subsonic filter is used to cut out rumble that you might hear from a vinyl record player if it's picking up vibrations. Leave it off if you don't have a record player.

As for the sub cross-over, it's very room and listener dependent. Do not go higher than 80 Hz. When I had M80v2, I crossed over at 80 Hz for movies and 60 Hz for music.

You can put the 500's volume half-way up and your Denon will adjust as necessary. I suggest you start by turning the 500 up about a third.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated