I’m surprised to hear you say that you like the depth/soundstage on the M50X! That’s not an opinion I’ve seen too often. Usually people say it’s very forward/in your face. I think it’s decent, but nothing special. Jealous that you’ve tried the M70’s, never quite talked myself into springing for them. I prefer the design/build of the M50. I’m skeptical of how my large head would work with the more plasticky headband on the M70.
The Arctis has a good sense of depth, though I haven’t done any lengthy binaural tests and probably couldn’t do that for some time, sorry. Even the base model of the Arctis includes decent virtual surround software for use on PC. The Arctis 5 comes with DTS Headphone:X, which is probably my favorite surround virtualization software.