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This isn't an answer to your question but a clarification. The USB port on the Airport Express isn't an output, it serves only as a connection for a shared network printer.
The output of the Express is a mini-optical jack. You can stream wirelessly from the Mac to the Airport Express, then go directly from it to the DAC 1 by using an optical cable. The direct USB connection you're after is supposed to offer higher quality but you could try the setup just with a cable and the adapter for the mini-optical port. I've been happy with a glass cable I bought from a member on this site, Captnstarstripe.
Gmood1: Thanks. This might just be what I am looking for.
Edesilva: My AX only has a 1/8" stereo mini out.
Sfar: Not sure if your statement about the USB port is correct. Gmood1 sent a link to "trends audio UD-10" that seems to convert the USB to other connections that go directly into the DAC. I'll have to look into this.
Zippyy, I was responding to your original statement that the AE has only USB out.
The devices Gmood1 sent links to don't connect to the USB port on the Airport Express, they connect to the USB port on your PC or Mac. To get an audio signal from the Airport Express you can plug either an optical cable or a stereo mini analog cable into the single audio jack on the AE. It's a dual-purpose port and either an analog cable or optical digital cable will work as long as you have the right size connector on the cable. The USB port on the AE does not provide the same audio-out capability as a USB port on the computer.