maxxaudio - how do i tell it that i have external speakers?

In the maxxaudio settings there is a place to select headphone size, but i am not using headphones, i have a cable that goes from my headphone jack on the laptop to my Cerwin Vega CV-1800 Amplifier which is in turn hooked up to 2 fifteen inch Cerwin Vega speakers.

I cannot click on bass boost because it is greyed out, and when i hover the mouse over it it says that bass boost is only available for use with external speakers. SInce i AM USING EXTERNAL SPEAKERS, how do I tell maxxaudio that i am using them and not headphones?

thanks