iTunes encoding to AAC creates distortion... Or not.

Hello:

A client of recently purchased some new high-end headphones and noticed distortion in some of the audio tracks in his iTunes library, tracks that he encoded himself to 256 AAC with iTunes. I heard it too, it was quite obvious.I proceeded to encode the original AIFF to AAC with TwistedWave (LAME engine), played it back with TwistedWave comparing with the AIFF, and the files sounded identical with zero distortion present. So I figured iTunes was causing the problem during encoding.

BUT, the distortion is STILL THERE when you play the AIFF file in iTunes, in the same places.

He reported to me:

Strangest thing. After you left I threw out the 256 aac file for Midlake’s “Roscoe” that we found to have distortion (encoded in iTunes). I tried to replace it with the aac file that we made with Twisted Wave and iTunes would not import it. So I imported the full res aiff file that I had and iTunes had no problem importing that. But when I played it back, the distortion was in it again, in the same places. I did a “get info’ and sure enough, it was still a big aiff file. When I play it through Twisted Wave, there is no distortion. But when I play the full size file in ITunes, the distortion is there. Why do you suppose this is happening?

Both files are being played back through a Focusrite Scarlet 2i2 D/A, identical signal paths.

This is the current version of iTunes on OS 10.9.2. 😕