That is a fairly nice receiver. If I recall, it goes for high efficiency instead of high out-put... if paired with high-efficiency speakers you will have one heck of a nice sounding across all volumes, and very loud, setup.
I'm using Optical out via my on-board solution, which is feeding into my JVC RX-5030V. Right now I'm only rocking stereo since I'm kinda' in a temporary living situation. However, I was using anolog and the difference seems night and day. I was using analog cause I didn't want to purchase an optical cable (only form of digital out my mobo has), but just got tired of it and finally broke down and bought the cable- was only $20.
My advice- considering the speakers you're using, the receiver they're hooked up to, and the fact you'll be getting blu-ray, I *highly* suggest you go digital. If the Audigy 2ZS has a digital out then that's one option, if not then just use the on-board. You'll not regret it.