Subsonic filter should go lower than 35 hz... if it only goes that low, then set it as low as possible.
Switching the phase will switch the phase of the signal. It's a switch, not a variable knob, right? If it is at 0 degrees, the subwoofer will move in phase with your other speakers. If you switch it to 180 degrees, your sub will move in when your speakers move out, and will move out when your speakers move in. Assuming you have no other phase controls switched or speakers wired backwards. It is always worth switching it back and forth and listening to which way sounds better in your car. It will be different in every car.