Actually, you can make a "decent" set of 5.1 headphones =) take a good set of headphones, make them slightly larger, add "small" rears and small fronts, and with some good emulation you can simulate 5.1
Except of course that doing this will probably wreck the acoustics. There's more to it than just what model and size of drivers you use.
Also as the ATHs are bloody huge to start with, the result would be rather...um...