I own both the Master Tracks and the XC model. Whether or not the XCs are worth the extra outlay really depends on what you are looking for from these cans. The XC's sound signature is much more neutral, clean, natural and less nuanced than the more mass market sound of the original Master Tracks. It's still got a slightly exciting, quick edge to its sound, something that works well with a lot of genres, but frankly, it's actually not ideal for music like dubstep and bass heavy EDM - ironic given the association with Calvin Harris. Having said that, it's actually quite responsive to tweaking with an eq or an amp - some headphones retain their nuanced sound no matter how much you work the EQs, my main criticism of the original Master tracks was that no matter how much you played with the eq bands, they always leaned on the bass. The originals had really strong distinctive bass, that tended to overwhelm in some tracks, this is where the XCs really trump them to be honest - the bass is not as intense, but the mids and particularly the vocals and higher registers sound a lot better than what many headphones at this price point should be expected to. The relatively flat sound signature also sits inside a slightly more spacious sound stage than the original Master Tracks, so with the better clarity, you also get better separation which actually led me to conclude that I liked the XCs for classical, jazz, rock, fast electronic and vocals much more than the originals, but would probably lean towards the originals for dubstep and EDM - ironic, isn't it? Nevertheless, for those who want a set of everyday cans that can double as decent monitors, the XCs are a great option.
They seem to me a lot closer to what would happen if you try to take an audiophile type headphone and make it suitable for life on the road.
I do think the Calvin Harris tie-up will worry some people, and it might actually pull in the wrong people - those expecting that over the top bass feeling you get at EDM performances will actually be a little disappointed unless they have a good amp to push up the bass, everyone else who isn't a banstead will find them absolutely superb out of the box and even better when fed a good source.
By the way, the "pro cable" that it comes with is a very very good accessory, the design is rugged yet elegant and thankfully everything bar the drivers themselves is replaceable - something that many other companies would do well to learn from.
They might seem a tad pricey, but it's the first multi-tasking headphone that might actually make audiophiles smile.