So I got a nice small package in the mail today... Cut the story short, the new case is about half of the size of the M-80's case which makes it quite portable. The pads have changed, the padding on the M-80 is more rounded off while the padding on the XS is flatter (this may be because of how they are packaged in their collapsed position). Either way, the XS pads have more surface area that will touch the ears, but also feel a lot more comfortable on the ears. The headband has also been changed slightly allowing more headband to actually rest on your head improving distribution of pressure (not that the M-80s had that much pressure to begin with) which should improve comfort. If anyone had any headband issues with the M-80, the XS should fix it up.
As found sound differences. Midrange differences can't really be heard through my initial impressions, however, there are some minute changes in the bass and treble. The bass is more linear overall but also more controlled and has better perceived depth. The mid-bass hump that the M-80 had isn't as audible any longer, however, the M-80 has better bass presence overall. The treble follows suite with more linearity as well as a little more detailing and slightly more extension. The differences are minute though, but Val did listen to our main critiques about the M-80 and did successfully improve it.
Also another note, not that it'll matter for most people, the XS does lay a little better around your collar compared to the M-80.