I'll clean up after JohnK.
The audio/video switch indeed gives an extra boost to mid-and-low bass frequencies to give movie soundtracks more impact (when switched to video). Some people like this effect, others don't. I would suggest setting your sub gain exactly where you like it for music. If you're not getting the impact you want watching action movies, flick the switch to "video" for some added punch.
Adjusting up individual levels for each speaker should be done with caution. Remember, for a +3 dB difference, the receiver will need to send double the power to that channel. For example, setting the center channel or surrounds at +3 dB can cause your receiver to clip if you turn up the gain so that the mains are loud enough.
Make sure that all of your speakers are set to "small" in the receiver setup menu. This will route bass information to the sub which powers itself. The Marantz should have no problem driving the small speakers in your setup to an adequate listening volume, as long as your room isn't too big.
