The possibly confusing interface Grant, may be coming off the computer. You can have one of three different solutions basically:
1. A USB DAC like the Apogee that takes the feed directly from your computer's USB outlet, converts the signal to something the DAC can understand, and spits out an analogue signal.
2. A USB interface device like my Waveterminal U24 which simply converts the USB signal to either Toslink (optical) or S/PDIF, and which still requires you output that signal to a separate DAC or digital-in on a select few one-box players.
3. A wireless device like the Airport Express, which can act either as a rather poor DAC and feed directly into an amp, or as a convertor device providing, in the case of the AE, an optical digital feed for a separate DAC.
It looks like the Offramp you suggested falls under category one, since it is a USB DAC essentially.
Drubin - thanks for the correction on the maximum USB length. Sorry for the misinformation.
Marco