HRT music streamer & Airport Extreme

It doesn't work that way to my knowledge.

I believe it will work that way.
The apple airport express outputs audio through USB and the HRT is a USB dac, so I think it would be fine!

Even the entry level products from HRT will be SIGNIFICANTLY better sounding than the USB output of a macbook.

Best of luck ***disclaimer, I am an HRT dealer, but I also sell a lot of other USB dacs and dollar for dollar the HRT are exceptional.

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Buff - I think you should check the specifications of the Airport Express.

The USB port on it is for printer output only, the only audio output available is from 1/8 stereo mini jack, either optical via a Toslink cable or analog with a 1/8 mini plug.
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The AE also has a gigantic JITTER problem with the digital output.
I'm not a fan, but I suspect a 'reclocking' device would help.

If I had it to do over again, I'd buy the Apple TV.....

Don't forget the 'mini toslink' adaptor. Don't worry about 'audiophile' here, since it is HOLLOW. And worth a buck. tops.