Hi, I have the same audio setup that you are looking for - except that my headphones are connected via mini-jack, not usb. I have 5.1 surround sound speakers and headphones permanently connected to my soundcard. I have put a single male to double female mini-jack adapter into the normal audio-out on sound-card, into which I have connected both the main woofer mini-jack (the five speakers are connected into the rear of the main woofer) and the headphones mini-jack. I can turn off the speakers from the controls on the front of the woofer and but still have the audio through the headphones.
This may not be exactly as per your own setup, but maybe just to give you an idea for yours.