As I understand it, you are also using an external subwoofer. If this is the case your main speakers will need to be set to "small", otherwise the sub will not receive a signal in stereo mode. If you are not using a subwoofer, you'll have to set them to "Large". Based on Polk's specs of a +/-3dB at 30Hz, you should use either the 40 Hz or 60 Hz setting (whichever sounds better to your ears- with your subwoofer). Feel free to drop us a line if you would like to discuss this further.

Regards,

Scott