AD700x and AD900x. Both were congested and grainy out of the box. As in, "these are horrible and how can anyone listen to these" congested and grainy. I was listening to a live recorded concert with a loud crowd, and 30-60 minutes of that mentally wore me out since my brain had to cancel out the grain and noise to pick out the notes. I heard sounds from the crowd I don't remember hearing before, but it was like the gain on an amp was turned up and boosting detail but also boosting grain and noise. I thought, "This sucks for music, but this must be why gamers love these. I can hear everything."
I let them burn for three days. They opened up and got smoother. The 900 has a smoother signature. With some eq, the 700 sound very close to the 900. People sometimes say the 700 has no bass, but it's not like the 900's bass volume is anything great. I haven't seen a comparison between the AD700 and AD700x other than some quick comments like saying the old AD700 has more air. Comments like that make me wonder if the AD700x did get a bass tweak.
They're airy, and the sound is diffused, which means the imaging sucks. Which makes me surprised they are so popular with fps gamers. With various other headphones, if you're watching a movie, and someone shoots an arrow or something, the sound comes from right there. With the 700 and 900, the sound comes from "somewhere around there."