Ah! ok.
Then you have Dolby Headphone. If you are unsure how Dolby Headphone works, it uses algorithms and audio cues to trick the brain into thinking the sound is coming from a specific location using just stereo headphones or headset. With 5.1 headphones like the Kave's, they use multiple drivers in each ear cup, so they will work with any sound device that has 5.1 analogue output, unless they are the USB type, that have their own built in sound processing. 5.1 headphones do get quite a bit of stick from a few people on here, me included. I think if one has a sound card or onboard audio that has Dolby Headphone, or CMSS 3D in Creative's case, and decent stereo headphones or headset, it's a better combination than 5.1 multiple driver headphones. Some don't think so though, and are adamant 5.1 headphones are true surround sound, and better than virtual.
There have been quite a few users who have gone from 5.1 headphones, to a gaming sound card and a stereo headset or headphones, and are glad they did so, finding that Dolby Headphone or CMSS 3D, is more accurate, and provides better sound quality.
I have an X-Fi, which uses CMSS 3D and stereo headphones. I also play L4D, and find the positional sound is great and works very well. I also have a Xonar in my media PC, which has Dolby Headphone, same as your D2X. I don't use it for gaming, but I have tried it with L4D a few time, just to see what Dolby Headphone is like, and again using my headphones, the positional sound is great.
One big difference between 5.1 headphones and stereo headphones is the sound quality. As 5.1 headphones use multiple drivers, it's common sense the drivers in a decent pair of stereo headphones or headset will be of better quality than of those found in 5.1 headphones. To use more drivers, they will have to be cheaper, and thus sound quality will suffer. A decent pair of stereo headphones/headset will have better dynamic range, which will produce better highs and lows.
I think it would be best to get some more opinions of the PC360's, whether at OCuk or another site first, if you are considering them. They are a hefty chunk of cash for a headset. Best to find out first if the price is justified.