Nice review thanks!
So it is all about the Bass.
I do a little of recording and mixing myself. I don't really get it why people often associate thin sounding headphone with studio monitoring. I know that I cannot use bassy headphone for monitoring, as the bass will be super annoying when I try to hear details. But it doesn't mean that the bass has to be thin either. From my own experience, monitoring headphones must be good in detail retrieval, but not necessarily thin sounding. Especially for mixing and mastering, the monitoring headphone should sound realistic enough, and that means sufficient bass (not too much).
Anyway, M50 also positioned and professional monitoring headphones, and already used worldwide. And it has good bass. So nothing wrong to have good bass for monitoring headphone, in fact it is a must.
It would be really useful if the review could mention, roughly, how many dBs is the bass differences between the M50 and M70x. We can use EQ and see how much bass adjustment required to equalize M50 and M70x bass.