Welcome to AT.
The first thing I would suggest is don't bother.
Your computer has digital sound to start with and at some point before it gets to the speakers, it has to be converted to analog for them to play. (by DACs)
If you output digitally, the DACs in the logitech control pod will take the digital input and convert it to analog.
If you use your Audigy2 ZS to convert it to analog, you'll be using the DACs in the audigy which should be of higher quality than the ones in the logitech set.
If you use analog, you'll also get the added benefit of true surround sound in gaming, and hardware produced EAX processing from the Audigy2.
Is this the card you have?
It does have digital out. One of the jacks outputs a digital coaxial signal that you could use. You would need a cable that converts a 1/8" jack (mono I think, you should look this up before buying anything) to RCA. Then run a digital coaxial cable to the logitech set. The Audigy would then output a stereo signal for everything except something that's DD encoded (like a DVD). The digital stereo signal would only be played on the front left, front right, and sub unless you have the logitech decoder manipulate the signal. The logitech set could do Dolby Pro Logic on it and get it to play out all the speakers. This would sound fine for music, but in gaming it's not going to give you the true surround effects you're used to. DVDs would be very nice though, as they are encoded in DD and you would get the intended playback on your z-5500s.
Having an audigy2ZS do digital out is like buying a sportscar to drive to work in heavy traffic. It really shines in analog out mode for gaming. It's not going to do anything better than a
Chaintech AV-710 in digital out mode.
