I'm a bit confused by your question. But then it's Friday and I'm usually confused by then...
The Squeezebox has no USB audio output. It does have Coax digital.
If your point is to compare an i-Tunes output from a PC going into a USB DAC with an i-Tunes output transferred wirelessly to a Squeezebox and then outputted to a conventional DAC I think that you also have to factor in ergonomic and economic issues.
Is this a desktop or room to room distribution system?
What are the quality of your source digital files? If you're using lossy files then the question is largely moot as the files are the weak link in the sonic chain.
There are several different technologies and chip sets for translating USB to SDIF. Some claim better jitter reduction capabilities but I have not compared individual chipsets as they are all part of complete DAC packages so comparisons of the individual methodologies is not possible. Some manufacturers claim they have made comparisons and their final choices were the best, but well, they would say that, wouldn't they?
USB DACs have a very specific application - offering an alternative to the internal USB to SDIF in most computers. Whether they offer a superior sonic option to a SPIF output into a DAC without USB (or a particular DAC's USB vs SPIF inputs) depends on the particular computer, DAC, cables, and finally the listener's personal tastes.
YMMV and usually does....