Steve,
"Async USB requires USB 2.0 compliant ports on the computer"
Shouldn't the rest of this sentence read as follows? "....if resolutions in excess of 24/96 are to be supported"
For instance my Ayre has a dip switch option (Rsrv A) for USB Class 1 where it still runs in Asynchronous mode, but is limited to 24/96. 24/196 requires Rsrv B setting for USB 2.
There's also this:
http://thewelltemperedcomputer.com/HW/USB_DAC_AsyncSlow.htm
Best,