Connecting Sprout to Airport Base Station via USB

I've been enjoying my new Sprout using multiple inputs for a week now with no problems hooked up to a pair of Micca MB42X speakers.
I just used the USB input for the first time to connect to my Airport Express to play through Airplay and got a strange error message that the connected device (Sprout) requires more power than the Airport Express can support. No music will play through Airplay.
The USB input (and the very same Audioquest Forest cable) works fine from my Apple Macbook. But when I connect the USB cable to my Airport base station (to use Apple Airplay), I get an error message saying the Airport does not have enough power to support the connected USB device.
Note that the analog input (3.5mm cable to Sprout input) works fine from the Airport, and allows me to play music through the Sprout over Apple Airplay It's only the USB that gives me problems.
Anyone have any ideas? Thanks!
Edit: Airport Express from Airport base station

Hi, I just saw your post while researching a related connection issue. I have experience with both Airport Express and Airport Extreme routers. Unfortunately, neither router’s USB port supports analog or digital audio output. The USB port will only support a shared printer or a storage drive. The Airport Express optical output can be converted to a digital coaxial output using a SP/DIF converter such as: with a minimal cash outlay. You may be able to find a converter that will output via USB instead. As an aside, when both the USB and Coaxial digital inputs are used on the Sprout at the same time the USB will take precedence.

I previously used a Edirol DAC with an direct optical input with an Airport Express. My experience was that it worked pretty well if few other wireless gizmos were in use on the same network, otherwise frequent sound dropouts occurred, I have friends who had similar issues when using an Apple TV for this function. Setting the Airport up to function as a separate network not connected to the internet provided much better results for me in terms of dropouts. This however came at the cost of no internet connection when my Mac was using this separate network - not a solution if trying to do work on the computer at the same time. If using the Remote app on an iPhone or iPad to control your music the network needs to be switched on the device as well - not a very elegant solution.

Regarding your error message, I receive the same message if I try to connect my iPad mini to the Sprout using the Apple Camera Connection Kit. The Sprout is odd in this regard as I have never had this issue when using it with my Cambridge DAC MAGIC USB inputs. The Sprout apparently does not have the typical 5V power supply available through USB to power the CCK. The resolution for the iPad issue with the Sprout was to interpose a powered USB hub. This is not a solution in your case however, since there is just not USB audio support on either the Extreme or Express routers. Hope this helps.