Thanks for the low-cost tip.I've found a gizmo from EPOS | Sennheiser has made me very happy with rendering 7.1 through stereo headphones: their GSX 1000 Audio Amplifier. This thing's intended for gamers, so there's an intercom channel that's easy to ignore. On the Mac you do need to delve into Audio/MIDI Setup to get the Mac to understand it takes multichannel on first use; after that, it needs no special prep.
No doubt the Smyth A16 Realizer is far superior, but I get full surround (including rear channels) that's entirely satisfactory for casual listening or movie viewing from the GSX 1000. Evidently it's got a Cyber Monday deal going for it today... $189.00
Sure, it's running on some population-averaged HRTF. That having been said, it works well for my friends and I. My expectation is that there will be a lot of more sophisticated devices at lower price points joining the A16 in the next year.