Network problem

Sort of tangetial to your question, but have you tried assigning the Denon a fixed IP address from your router? That's what I do for mine, which makes it much easier to access via a browser or the Denon iPhone remote app.

You'll want to use the Airport utility to check the signal strength between the Time Capsule and the Airport Extreme. In my application, I required two of the current generation Airport Extremes to bridge the distances since the Express didn't have the power to do it.

If you have sufficient signal strength and you are seeing a data rate that is, say above 10-20, I'd double check the Airport Express is set to extend the existing network.

After you verify all the above, I'd grab a laptop and see if you can use the ethernet from the Airport Express to get on your network. Of course, make sure you turn off your wifi on the laptop. If you can browse successfully using the ethernet that would indicate the network is working properly and the issue is with the Denon somehow. Oh, you might try the Airport Express in two locations, close to your Time Capsule and installed near the Denon. If it works in the closer location and not the farther one you've likely got a range\signal issue.

Range issues can be fixed by replacing the Airport Express with an Extreme. Or, your best option would be to suck it up and run an ethernet cable from your Time Capsule down to your Denon. After all, the need for a fast, reliable connection at your receiver will only increase. My theater requires a much more robust connection to the network now than it did when I installed it in 2005.