I know this is all a matter of personal taste, but I compared these headphones at the HeadRoom WOH tour, and I must say the 831's totally blew me away. They were my favorite phones there. I liked them significantly better than the 250-80's, as well as the Senn 580/600, for that matter.
I hear alot of complaints that the 831's are thin and don't have enough bass, but to my ears the sound was clean, crystical clear, and well balanced. I agree that this type of sound is best suited for Jazz and Classical. They had a Kenny Burrell CD going at the time and it knocked me out.
All the comparisons I made at the WOH tour had the cans running through headphone amps. At 200 ohms, the 831s appear to be more for home use than portable use. They are also quite large, but very comfortable. They would be kind of inconvenient to drag around with a portable. That's why I elected to go with the Etymotic 4P with a 4S converter cable.
Vince