[rhythm is life]

In my experience:
 
DT770 (barely qualifies as portable, expansive soundstage is natural and comes from large ear cups)
Bose AE2 (very portable, soundstage is fairly wide, cups are small for an over-ear headphone - there's a pretty significant dip at 2khz which can give the impression of a bigger soundstage)
 
You guys are making me want to upgrade from the M50 to the A900!!! I used to have the DT770 and the one thing I really miss from those cans is the big stage.

 

[GloriousLettuce]

I have not heard a lot, but according to my experience and reviews online one of the best value and soundstages are DT 1770 and Soundmagic HP 150.
 
I have HP 150 and I'm just blown away by their performance on a daily basis, with HD 600 and X2 in the mix they have an amazing soundstage, and sound cleaner than both which sounds riddiculous to say.

 

[ChaseM]

I have the K553, M50X, DT770 along with some summit fi stuff. As far as just soundstage is concerned, at that level I think the K55X's would be hard to beat. They really have a massive stage for a closed can. And they are built like a tank, all metal and can take a beating. Very comfortable too. They can come across as a little bass light at times but respond very well to EQ. I use them all the time with Spotify on my iPhone and just EQ the bass to taste. They're a great can.