Who mods class D/digital amps?

Try reducing the analog input sensitivity and see if that helps. Also, are you sure the IC's you have are balanced? If you can't tell, list them.

This problem doesn't sound like the type of thing you can fix with a mod.

I don't know how to reduce the analog input sensitivity. It may or may not be accessible through the onboard controller, but I would have to access the configuration via a PC. I haven't ventured into that yet. The distortion only occurs on certain CD's. There is no correlation between the loudness of the recording and the distortion, as far as I know thus far.
I assume you mean interconnects when you mention ICs? My preamp is not balanced. I have RCAs on one end of the cables and XLRs on the other. The Air20s have in input impedance of 3000 ohms for unbalanced and 10,000 for balanced. I have no idea what the output impedance on my preamp is (Tube Audio Design DAC/pre). I cannot find the specs on it. But you may be on to something, the impedance may be mismatched.

Thanks for your input.

You may want to check in the owners manual because there's no standard way to deal with this. Balanced components are usually designed in such a way that allows them to switch back and forth between balanced and single ended operation. So if your preamp is SE, you want to make sure your amp is set to run SE also. If not, you can have all kinds of problems. What you need to do will definitely be listed somewhere in the manual.

Here's a few things that may prove useful.

-Sometimes amps that are not balanced still have xlr connections. So you want to make sure that your amps are fully balanced.

-Same thing with cables. They need to be fully balanced and not just an SE cable with xlr connectors.

- Also make sure the CD's themselves are not the problem. Try them on a different player just to be sure.