I haven't heard the HD25 in a long time, but I've really liked it every time I have. I asked Sennheiser USA to bring a set to Can Jam '09, so I could hear it again. While there, if I remember, I would like to compare the two (SRH840 to HD 25-1 II).
It's been a while since I listened to my hd25 in a good rig (use them at work currently straight from the computer *cringe*) but the soundstage wasn't very large as far as I remember. Nice if the Shures can do soundstage better.