Good to see a couple of useful responses. I've narrowed it down to the DT-880s and SR325is, but am having quite a hard time choosing between the two. It's unlikely that I'll get the chance to hear either before I buy, and will definitely be getting rid of my HD600s once my new cans arrive (I want to compare them first). It's just that I really don't know which ones to go for.
325i = bright, very detailed, energetic (fast), smallish sound stage (ie you are up there with the musicians), bass is punchy - but mostly mid-bass, forward mid-range and upper mids, if you're not used to Grado fit - can become uncomfortable. Can be driven from almost anything.
DT880 = bright, very detailed, a little slower than the Grados, medium sound stage, bass is well textured, extends further, and is relatively balanced. Very neutral cans. Some say mid-range is recessed - I say it isn't, it's just not forward like the Grados or HD600. Very, very comfortable. Assuming you get the 250 ohm - really needs amplification. Only go 600ohm if you have an OTL tube or reasonably powerful SS amp. Honestly 250 ohm is more versatile anyway.