I posted this somewhere else as I got asked this question before:
I only had the L2 for 1 day, so I can't say too much about it. But if I recall correctly, the mids and highs were a bit more forward on the L2 and the bass was a bit more present. So overall a less neutral sound signature, a more warmish bass and somewhat more pushed mids and highs (not overdone though). The bass was still fairly tight though, but I don't think it's as tight as the 7520. It was actually quite an enjoyable headphone to listen to, just that the earcups were too small for my ears. In many ways they sound better than the P7, which is also a great headphone. And also I was really looking for a closed headphone, and the L2 is semi-open. But that openness certainly helped with creating a very expansive soundstage, if that's what you're looking for. Oh and in terms of design I think they're the coolest looking headphones of the whole bunch.
And to add to that, I remember the highs on the L2 being very crisp and having a bit more presence than on the 7520 (I guess some might call it a bit bright).