Sound Card Conundrum

I recently bought a pair of Bose Companion 2 Series II speakers for my new computer, a custom computer I built from the ground up (+/- $1,800 Gaming/Multi-Purpose Rig).

The on board Realtek ALC898 of my ASRock Z77 Extreme4 LGA 1155 supports a myriad of input devices, but since the new speakers I bought do not have a subwoofer with them, how would I connect a stand-alone subwoofer to work in conjunction with the speakers I already have?

Most subwoofers I've seen have RCA inputs, the clamp style speaker wire inputs & outputs, and some appear to have what seems to be a 1/8" input or output jack.

Is there anyway I can use the peripherals I already have at my disposal to connect a subwoofer to my rig somehow?

Would I have to get a new soundcard that has some sort of compatible peripherals on it or are there some special cords or adapters I would have to buy?

Please guys, I've asked many people through many different channels & no one seems to have a clue.