This is probably a stupid question, but I'm going to ask it anyway!
I know that the Z1000 and 7520 sound different, but why? I think the pads are slightly different which could make a small difference, but there must be more to it than that. Do they use different drivers, or are the drivers 'tuned' differently in some way?
Someone did an analysis on this a while back, perhaps even in this thread, or it could have been a different website. But yes, the drivers are tuned differently and they're also physically different. The drivers use some white material (don't know what type of material it is) and I believe the thickness of this material varies between the 7520 and the Z1000. Also there is a little hole (perhaps a bass port?) in this white material, which is larger on the 7520 vs. the Z1000. So the drivers are physically different and hence sound different.