Everyone is different.
I force myself to have only one pair of headphones at a time, for budgeting reasons (sometimes not easy). I haven't heard the HE500, but let go all the rest you have mentioned, and ended up keeping the X00s. (Plus let go some other hps like HE400s, NAD Viso, Nighthawks)
M100 is a good portable bass monster in a good way. But when it comes to mids and treble (especially mids) it lacks a lot compared to anything else on this list. This was the easiest good bye, however I did enjoy them.
HD600 is an eternal king of mid-fi opened back hps for its naturalness and spaciousness. Perfect solution for acoustic and vocal music. Not suitable at all for EDM. (Lacks bass extension a lot.) So this one has gone due to my music preference, while I still appreciate its strengths.
X2 was one of my biggest loves. It does so many things so wonderfully. For an insanely good value for money. Bass is about the best an opened back headphone can offer, and in every other aspect it is a more than fair deal. Also, very enjoyable, what not many opened back headphones can tell. (Yes, there is, a beauty in a completely natural and flat neutral curve as well, but only for vocals and acoustic instruments.) When electricity is involved in producing music on any level, this fun factor becomes much more important. And X2 excells in that regard, while it keeps the most of the benefits of an opened back headphone. (Space, relative naturalness.)
It was hard to let X2s go. But I really want to keep only one pair of headphones at a time, and in my case with my music preference X00s were just better. Bass is absolutely fabulous, everything is there what I missed with X2s. (Extention, sub-bass, slam, punch, tightness, clarity.) Mids are more than ok, and treble is shiny, yet not piercing. After bass, the second most obvious improvement is resolution. So much more details are coming through, while the sound stays very much enjoyable, but not analytically dry at all. You can hear details in the recording what you can't not hear with X2s, and all of this comes with a much better bass. (Thanks to the semi-closed design.) Yes, there is a little compromise in airiness and spaciousness compared to X2s. But the gain is much more IMO.
This is my story, my preference.
YMMV.
I am curious however, why did you let X00s go over X2s. You must have your own reason.