Its delightfully easy. Note these steps:
Plug in USB headset - Soundcard disables, headset works
Unplug USB headset - Soundcard enables, Z5500's work.
Only problem is if you do it mid mp3 winamp throws up and error, but simply start the track again and its perfect on your desired device
Unusual that it works so well, normally there would be some sort of conflict and the world would end, but nope works 100%
Enjoy your z5500's
btw, they are lovely. Just give them a week or 2 to loosen up, they sound fantastic when you first get them but after the 2 weeks oh boy do they get better.