Going through a catalog last night, I came up with a couple of additional options that you might want to look into:
Aardvark USB3
Edirol UA1X (this seems like the best option, to me)
Edirol UA1D
UA3FX
Edirol UA5
The UA1X sells for under $80, and is a 16-bit/44.1 kHz USB audio interface with stereo RCA I/O as well as digital SPDIF output (and a headphone minijack). The UA3FX and UA5 seem to offer better A/D-D/A converters, but might also incorporate features that you don't need.
And the more I look into it, the M-Audio Audiophile USB does look like a very nice unit, though again, it is packaging some features that you don't really need (though not to the same extent as the MBox).
Good luck, Tom.