I feel like the HD595s put a "veil" between you and the music. That may sound more accurate compared to 5th row seats in a concert hall, but I couldn't stand that veil!
In my opinion that is the only reason why someone should consider the HD595's over the AD900s - if you really like that "veil" and don't like the upfront presentation of the AD900s.
In my opinion, I prefer to feel like I am standing right in front of the recording studio (ad900) and the detail that comes with it rather than in the middle of a concert hall (HD595). Would you rather buy tickets to a regular concert or tickets to watch a recording in a studio?
The only time I might prefer the "veiled" style of the HD595s is when i am reading or something and would rather have the music sound like its coming from another room than right in front of me/around me.
And about that comment "the AD900s lack that "boom" that puts a lot of detail in bass"--what headphones give you that deep, quality, detailed, yet still have that "boom"?? I would want to try them.