I haven't compared the DT880 and the 7520 directly, as in A-B testing, but read on and allow me to exaggerate a bit in the following...
DT770.
Before acquiring the Sony 7520, I used the Beyerdynamic DT770 for years. I didn't know better, but looking back, the DT770 is useless for the occasional mixing I do on headphones. Yes you heard me, I find them useless for mixing compared to the 7520
The Beyer sugarcoates everything to give you a sweet ride. It's almost like it performs its own mastering process to the signal, it raises the bass and simultaneously softens it, it removes "unpleasant" low-mid body and high-mid frequencies, and it adds endless top-end treble and compresses it, to slow it down nicely. Sugarcoating.
DT990.
I borrowed a pair for a while. Still useless for mixing as it carries the same signature as the above DT770. It sounds better though overall. Better resolution and more lively / open sounding, but still useless sugarcoating everything. I think it has even more treble and bass than the DT770 if that's possible. Mixing vocals on these would turn out incredibly skewed on other playback systems.
DT880.
I have only tried these for a few minutes a while back, but they sounded much better to me. They had a more honest midrange and better leaner and faster bass compared to the above two Beyers. But it still had way too much high-end treble compared to the 7520.
Example: if you know the sound of a large crash cymbal, you know it has (reproduces) a fairly wide frequency range. It has both body and top-end sizzle. On the Beyers, the body of this cymbal is gone and all you are left with is top-end sizzle. AND it sounds like the sizzle is raised in volume to present it with lots of air. That makes it skewed compared to my preferred balance when listening to a raw recording of a crash cymbal.
Do not assume my comments applies to your tastes. Let's just say I want a boring sound, not a sugarcoated one.
A crazy thought: the Beyers might sound better balanced on older recordings (made before 1985). On todays songs, we have the amount of bass and treble covered, so you don't have to compensate any longer, Mr. Beyer
I can relate to many of the things said in this DT880 vs DT990 comparison.