Quoted from review of the SA 8003 unit by http://www.enjoythemusic.com/magazine/equipment/0109/marantz_sa8003.htm
"The USB input is powered by an NEC uPD63901 microcontroller (designed for use in car audio head units), and it enables the SA8003 to play MP3, WAV, WMA and AAC files directly off a USB hard drive, pen drive, or iPod (it does not function as a USB DAC, as connecting a laptop to the Marantz with a USB cable does nothing). The uPD63901 does its own D/A conversion, so files read from the USB input completely bypass Marantz' CS4398-based digital section (and cannot be output via the Marantz digital outputs). The USB signal still benefits from going through the SA8003's discreet analog output stage, but overall the USB sound is not quite up to par with what the SA8003 does when playing a shiny disc. Of course, this hardly matters unless you are using full-resolution WAV files, the only lossless format playable on the Marantz.
A more significant misfire is the user interface. Devices connected to the SA8003 via USB can only be browsed one line at a time on the Marantz' front panel display. This works well enough for a flash-ROM based pen drive with a few files on it, but it all but eliminated the convenience and joy of using my 60GB iPod. Only about one-third full, my iPod has over 180 artists on it... listening to one near the middle of the list required as many as 90 clicks on one of the Marantz' "folder" buttons, a significantly more cumbersome process than the iPod's own interface, which is disabled when connected to the SA8003."
Hi Amham, just wondering if the SA8004's USBs have the same limitation as the SA8003.