Hehe, they are amazing!
Just don't run them through Direct.
The sound is poor compared to Optical/Coaxial (not tried Coaxial yet, but its still Digital), but never run Z-5500's through the 3 jacks on standard sound cards, it just doesn't do them justice at all, ánd they sound closed and less vibrant.
You only need to plug them up to the Xbox 360/PS3 to discover the Digital connections are the best solution, when playing music especially.
If you only have teh three jack connections, and don't have optical outs, unless you are buying a new sound card, I wouldn't bother buying the Z-5500's.
Honestly, I can tell the difference, and spending so much on a powerful system like that, to hamper them, is no good at all.
I'm going to get the Coaxial working on my X-Fi soon... I hope, or I may just carry on streaming music through the 360.