Pros: Bassy, nice design, high isolation, folds up compact
Cons: Plasticky build, very high clamping force, not very breathable, difficult to wear around the neck
Along with the summary below, I have posted a Youtube video review of the XB900. If you like the video, check out my channel for more reviews :)
The XB800 has a very chunky design which looks pretty neat and offers high isolation. It is also easy to adjust and folds up into a nice compact shape However they have a very high clamping force and are somewhat uncomfortable. They are difficult to wear around the neck because the cups prevent you from moving your head too much. The build is overall a little plasticky.
If you wanted bass, you get bass. There isn't much else to say - the bass is deep and tight and the rest of the frequencies are severely rolled off and somewhat congested. Overall they sound veiled and muffled. I'm not sure who would want this much bass but I suppose this would be someone's cup of tea.
I can't really fault the XB800's sound since it does exactly what it says on the box - gives you a kick in the ears. However it isn't exactly the most articulate of headphones and the design leaves a little to be desired.