I demoed the K550s and AD900xs for a week. I eventually chose the K550s. Sonically, they were both great, but the wings on the AD900xs were annoying. Walking around with the AD900xs meant that I was constantly re-adjusting them due to the fact that they were sliding down on my head. Getting a good seal on the K550s is a bit tricky, but once you do they stay put and sound great. I actually went jogging with the K550s once (forgot the earbuds at work that day).