Thanks for the information, especially re:Waves Nx not being for real. The Rode plugin looks very nice. At this point, C10 has ambisonic support, so matters are presumably simpler.
I like the idea of music not being headlocked, unless there’s a more important audio environment going on at the same time, such as a gaming situation. It would be more interesting to hear the music slightly differently each time, or to subtly alter the mix by moving your head.
I don’t know how the head tracking would work without video. The playback software would presumably ignore head tracking data in the absence of a virtual (video) environment. I wonder if anybody is thinking about this. You could be browsing the web, say, and listening to head-tracked ambisonic music. The visual information would have nothing to do with the head tracking in a case like that.