i have no personal experience with it but based on the comments of 440 owners who went with the 840 pads, the 840 pleather pads are softer - thus making for a better seal (isolation is improved) and since isolation is improved the bass is also slightly improved.
plus these are softer than the stock 440 pads.
before you shell out $20 though try out the earpad mod i linked to a few posts back - it really helps :D my ears kinda stick out and the mod is a godsend.
oh and for everyone wondering if it is worth it to cut open your 440s - yes it is, my 440s have been getting way more head time now because of the decreased weight and the earpad mod.
i listen to a lot of jazz and rock and classical - DMB and Bill Evans (FLACs with a good DAC) are wonderful on the shures - and i have yet to find a closed (or open) set of headphones under $100 that provides this much clarity and articulation. my PX100s (i have Mk1 and Mk2), superlux 660s, and iems don't give me that "live" sound.
the drivers are simply too good for the money. Shure went out of its way in its first venture into the full-size headphone market to make a good impression - the build quality is questionable, but the drivers are good.
Edited by Mr Portable - 1/17/12 at 10:02pm