Probably a C. The mix is super frustrating because when I really focus in, there are parts that I quite like. But if I'm just listening to the song normally, a lot of those cool and interesting bits fall by the wayside. Basically, they've recorded a pretty good song, but they put the pieces together in a way where it's difficult to appreciate.
The mix and mastering certainly aren't terrible, but they leave a lot to be desired. Some elements sound muffled, and it leaves a decent song sounding less impressive than it could. I'm still super excited about the new record. I just wish it sounded better (I remember having similar feelings about DT12, Good music stifled by sub par production. Though this isn't as bad at least). It's especially frustrating because as complicated as my thoughts were on The Astonishing, I felt the production on that album was a big step in the right direction. This song sounded less clear than that.
I was listening to the song on a streaming service I subscribe to. After it finished, a DT song from the early/mid 90s came on. The clarity of the mix was night and day. Nuances and details were just so much clearer, and this was from the same source, so it wasn't a matter of low quality online video versus local FLAC or anything. It really struck me, and bummed me out a little bit since it made it pretty clear that Untethered Angel would probably sound leaps better with a slightly better mix.
I'm still greatly looking forward to the rest of the record. It's just a bummer that the mixing is what it is.