Canali, can't help you on the m70x comparison.
I did listen to a momentum the other day and it's clarity was sub par. This was out of Sennheiser headphone station at BestBuy, so not the best source but the HD558 next to it was much much better (though it's bass wasn't very good). I owned the HD555 many years ago and so I could tell that there as nothing wrong with the headphone setup at bestbuy as the 558 sounded great, just the momentum was tubby sounding.
I think I'd have to give it a bit more of a listen and I don't think it was the second gen momentum. The smaller momentum was worse.
For a closed headphone I've never been happier. This is after many many closed backs - mdr7506, akg k271, hd25, Amperior, srh840, Denon ah-d600 to name a few. Typically, I'd say there are trade offs with headphones in this price range but the 7520 handidly beats out most 'pro' headphones and consumer headphones that I have heard. I'd like to spend some more time with Focal Studio Pros, PM3 as well as listen to ones I haven't heard as yet such aS ZMF Vibros, Alpha Dogs, and Ether C.
I'm exceedingly happy with these, especially as this evening I took them apart to finally tackle the squeaking I had from the internal gimbals. Took a bit to figure out how to get the pads off (best to grab them from inside and push outward). The 7520 has a simple but sturdy construction internally.
Happy to report that most of the creaking issues are now resolved. Also, as of late, this headphone has become more comfortable to the point where I enjoy using it at work where it isolate quite well; well enough that I can barely hear my coworker next to me when he is on the phone.
Just really impressed with this headphone. I'd rank it up there with the venerable HD650 as it just has a presentation that you can engagingly listen to all music types yet without fatigue. Superb all rounder.
Listening to Handel / London Baroque - Emma Kirkby, not something that would typically work well on a closed headphone but was quite sublime.