As said in my previous comment I owned the hd800's and plan to get them again before the year is out. Although I had them for a month I listened to them almost every day.
Quick comparison between the 800's and x's (my dream duo at the minute)
Soundstage - 800's have the width and depth and excel in this department, but the x's do a much better job than expected and don't seem to exaggerate distance and holographic image a song can produce. They are extremely satisfying.
Imaging - placement of instruments goes to 800's with the x's close behind on many but not all genres i listened to.
Instrument separation - 800's again but with x's this department scoring seemed very close at times.
treble - I preferred the x's - detailed but smooth and never ever fatiguing. the 800's of course have great treble but a little cold at times.
Bass - while the 800's have a very underrated bass, the x's bass is phenomenal. Maybe too thick for the odd song and I'm talking no more than 2 songs (not genre) but for the most part just amazing, especially with drum and bass - jesus they were a dream.
mids- rated the same for different reasons - sorry to be vague but I have a hard time articulating this.
decay/speed - both are fantastic but the x's don't miss a trick and seem to be faster than the 800's
Comfort - This is a hard one for me. I find the 800's comfortable but after a couple of hours the headband becomes a little too present on the top of my head.
The x's surprised me. They weigh a lot as people said but the comfort was great, despite what most say (as it was with the 3's - i found them way more comfortable than what i expected given the negatives people say). The weight is distributed perfectly.
What I did notice after the long session of leaning over looking at my laptop that the weight of the x's had taken it's tole. This is by no means a deal breaker - all I had to do was sit up straight.
Genres (in short and based on my library)
Jazz - 800's do great as you all know, but enjoyment wise the x's were on par - technically, i actually preferred certain aspects such as trumpets and cymbals and the found the soundstage more than adequate on the x's
classical - and this one did shock me - i preferred the x's with violin based performances, much more, and overall i could happily listen to classical with the x's (very happily). the 800's of course do great job on a whole with classical and i don't own a high quality classical piece that the 800's don't make sublime.
rock - x's and it's not even close.
edm/drum and bass/ pop - x's and it's not even close. I dream of headphones sounding this good with drum and bass. It's like the 800's had sex with the 650's and made warrior babies and then they were adopted by Audeze.
Hip hop - x's and it's not even close
Reggae - x's and it's fairly close
On whole I enjoyed the x's more than the 800's with many more types of music.
I believe I will eventually own both to cover all bases, and truly believe the 800's are the king of jazz and classical, but my god do the x's just sound great with everything
Wrote this quick guys so please be kind
it's my birthday