Well i cant say i heard the XD400.
But i can say i been listening extensively to my A900.
I will try to help you answer your own questions and seek help.
What facts you need to know when deciding on these cans:
A. The two are NOT in the same class.
One is High end, for people wanting the audiophile sound and to spend a more than any commercial consumer would spend on headphones.
200.00
The other is commercial, approaching high end.
For the above average consumer, most I know that this price is their limit.
80.00
B. Anything that puts tacky words like "movie" mode means more to the commercial consumer. Any high end headphone doesnt have words
like XBS, Mega bass, Movie, Big movie sound, etc.
C. The XD400 i dont expect to be a total slouch, because there are many people who enjoy these cans.
D. What is your use for them? Movies, Games, music..and a ratio?
E. Ask if best buy will demo these to a receiver for you. They are commonly laying around on a table display to wear, but if they go that one more extra mile...they MIGHt do that.
F. A900s excel in Movies, all kinds of music, and Games.
I doubt you will dissapointed unless you want to spend more money or you want something even more laid back.
These provide to me the right balance between laid back and aggressively in your face.
G. When one buys a High end can, usually one don't want to go down the ladder unless they want a portable can or Iem, but something different. 2 cans for the same task with one in a lower price class only deters the value of the dollar.