I have not yet heard the MDR-7520 but plan on adding them to my collection at some point.
I think that both the Senn HD-600 and the Focal Spirit Pro have a sound signature that is studio ready and at the same time well prepared to wow the consumer music listener. I have both and have been in and out of the project studio hobby for many years. I am no pro, but have seen many in the biz speak highly of both, and my wife, teenage son and myself all find both to be outstanding with anything like a Dragonfly on up... The problem is that the former is open and a bit dated (not a resolution monster in today's market), and the latter is designed for smurfs rather than normal people. I have a small head and big ears, so the Spirit Pro is manageable clamp wise and a pure on ear experience for me and thus measured against other on ears... so I put up with them because I think they have such a great sound signature. I find myself preferring them to even my HD-700 in some cases when listening to music for fun. I also have a big smile whenever I put the HD-600 on my noggin. What a fantastic achievement from Sennheiser that still stands the test of time.
I want to add that when I say "signature" in this case, I mean very small deviations from "pure flat" and in ways that IMHO benefit the goals of a studio engineer/producer while at the same time not taking away for the more organic musicality preferred by the average music lover. I think that is a very rare feat. I don't think the MDR-7520 will do it but I need to hear them for myself. I know (just subjective here...) the HD-800 won't do it unless you are ready to go on the world tour of DAC and amplifier auditions to find the combination that works for you and your preferred music. My opinion again, but I don't think that any headphone with really rich and present bass in a way that appeals to many consumer listeners will ever cut it in the studio...though I hold the door open for a few ultrasone models that I would love to hear and judge for myself. I like this thread because my own personal listening preferences tend to line up fairly close to a studio pro, so I am always looking for a balance between pro and consumer SOTA, and then deviating just a little bit for fun, like with Grado 225 or AT M50.