Also, the condition of the ear pads is VERY important for several things. For starters, if the ear pads are greatly deteriorated, these headphones will leak sound. A lot actually if the ear pads are totally stripped, even though it's a closed back headphone. Also, if the ear pads are really deteriorated, the isolation from outside noise won't be nearly as good. In fact, it'll pretty much sound like you have on ear open air headphones on. But with ear pads that are in good shape, these headphones will block out much more noise.
Now for the sound quality. If your ear pads are totally deteriorated, the bass on these headphones will be abysmal. There won't be very much of it at all. Having a good seal is very important to good bass with these headphones, and if all you have is the foam on your ears instead of the pleather, then you won't be getting much of a seal and the sound will have a less focused and less powerful effect due to less bass.
With some headphones, they build so much bass into the driver that you don't need a good seal (koss porta pro). Hypothetically, if you put the Bose on ear ear pads (the really nice memory foam with the real leather covering) on the porta pros, the porta pros bass output would increase a LOT, and it would be pretty unbearable actually. I've sort of tested this out with different headphones...it makes a pretty big difference.
One thing that helps prevent the pads from dying an early death is using electrical tape on the outside of the ear pads, the part that goes around the ear cup. It holds em together a little longer.