I really like my 840s. They sound great and seal out external sound quite well. I can't find anything wrong with the 840s, it's just that I got a tax return and am curious about what kind of great sound I can find in the $300 range. :)
The closed aspect is important to me, as these will mostly see use on my gaming PC, which is a bit on the loud side (too many fans). For example, I have Denon AHD2000s, which are sold as a closed design, but they don't seal that well. They sound great, but just aren't ideal for use on my computer.
I'm not interested in any sort of active noise cancelling headphones.
On PC the new headphones will be used with a FiiO E7 DAC and E9 headphone amp. They will probably see some use in my main music rig as well, which is a CD player, Schiit Bifrost, and Asgard 2.
I've looked at the Shure SRH940 at $275, and the NAD HP50 at $300. Then there's the V-MODA M-100, also for $300. I'm also curious about AKG, Audio Technica, and Sennheiser, but I'm not all that familiar with their closed models.
Accuracy and detail are things I like in headphones. I like to hear exactly what's on the recording, or all of the fine little details in the sound of a game. A good soundstage is a wonderful thing, though I don't expect a ton of that in a closed headphone.