I had to try to enjoy the Denon and never had to try to enjoy the M2. It is great how we can have differing opinions on the same hp's. I always kinda felt I missed out on something by not hearing the P7. But to me the M2 was clearly better than the MM400 and out of three pairs of the M2 I never detected a hint of sibilance. I also think the build quality is top notch.
Indeed that's why it's awesome that we have so many great choices. As I said, you can't go wrong with any of them. If you didn't have them side by side, it's hard to pick up the differences. at first listen, I was the other way around. I was keeping the M2 and the MM400 was going back. Then I burned in the MM400 overnight and started using them daily. The M2 also started to pinch my ear against the arm of my glasses. For whatever reason, that never happens with my P7 or my MM400's.
I hope you get a chance to try the P7. They are also outstanding. Enjoy!