>5.1 -> headphone HRTF implementations

The technology I tried is intended to be used for real-time games. Individual game objects' audio samples and their positions are run through their filter, accounting for position and the material properties of nearby reflective surfaces (hard vs soft, solid vs semi-solid).

Is this fundamentally different to the implementations you've tried? It seems to me you're referring to '3D' implementations that take an already-mixed 4 channel source and try to jam that into a spatial 2 channel mix. That's the kind of technology that I think is pretty lame/useless as well...

If you have a chance to try out Visisonics tech, I think it'd be worth giving it a go if games are your thing. Seems like you definitely know a lot about the subject.