I have been told that the USB outputs on the Macs and PC's are limited to 48k/16 bit
Yeah, I don't believe that's true. My understanding of this is a bit weak, but I have looked at the
USB 2.0 specification download, and it seems if you refer to endpoint synchronization characteristics for both source and sink devices on pages 72+ of the usb_20.pdf documentation, you can see the maximum data rates are determined by several factors and are by no means limited to 16 bits or 48 KHz.
I have come across (on the internet), devices that load their own Windows drivers to take care of the basic maximum native Windows USB data rate of 16 bit / 48KHz, but I've also seen devices such as the
Benchmark USB DAC1 that claim they offer USB input support up to 96 kHz sample rates at word lengths up to 24 bits, while still using the basic native Windows USB audio drivers. This seems appealing to me as a plug and play DAC without messing with Windows. I would think that most of the usage with respect to the USB ports for external DACs would come from Windows computer driven streaming music.
Presently I use my laptop for streaming audio out to a standard 16 bit / 48 KHz bit USB soundcard, and then digital out from that soundcard to my Bryston SP2 processor.. I can't use an optical SPDIF output from the laptop, because it only has an HDMI output from the laptop (since it's brand new). HDMI interfaces on laptops will become more and more prevalent as time passes, and so USB will become (I would think) the output of choice for streaming digital music to feed an external DAC (or USB soundcard).
If I ever purchased an external DAC to feed USB streaming audio (to increase the quality of my present situation), I would hope it had the capability of greater than 16 bit / 48 KHz transfer. It seems to me that the ability to pump that up to 24 bit / 96 KHz is certainly possible, but maybe I'm wrong.......

brucek