Who mods class D/digital amps?

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.

It's a pretty drastic sort of distortion, effective throughout the full audible range (to my ears). Think of a clock radio with the volume turned up to the full level (this is a bit of an exaggeration, but you get the idea). It is only evident on certain CDs/tracks.

I spoke to Dynaudio, they really recommend a balanced preamp. When I use the digital AES/EBU input the distortion issue is nonexistent.

By the way, in response to the other thread, I'm not very technical, however, I'm not new to audio or Audiogon by any means. Joel at Dyaudio referred to the Air20 amps as digital amps, if you care to debate the semantics with him, feel free to do so.
Also to clarify, my goal with audio gear is accuracy. I neglected to mention this in my other post, and perhaps due to this omission, there was some lack of focus. For this reason, I generally believe that newer gear is "better", in the sense it's more accurate. Now, don't get me wrong, there's nothing wrong with a pair of old JBLs with McIntosh tube amps driving them. They sound lovely, just not what I'm looking for. I realize that the gear that comes out every year is not necessarily an improvement over the past year's, however, if one were to plot the overall progress of audio over the past 50 years, it would be hard to deny the general progress in accuracy in reproducing the audio signal.
By the way, thank you for commenting on the specs of the Air20s. That is really all I was looking for; to know if the specs were up to date. I was just curious. It's not like I was about to trash them if they were not. They sound amazing. I trust my ears.