Not sure exactly what you are trying to do.
You want the "option" you use your headphones OR your 5.1 surround system separately from each other correct?
Easiest way is to connect the analog 5.1 outs from the motherboard/sound card to a 5.1 input on your receiver/preamp. Of course you will lose out on any Audyssey or other types of room correction unless you use something else downstream.
As for your headphones your best/easiest bet for surround sound is dolby headphone via your computers front panel analog headphone input. When plugging in the headphones it will mute the rear analog out signal.
If this will not work for your setup then it starts getting a little more difficult and or expensive and can turn into an audio rabbit hole pretty fast.
