You are making an assumption that what you do not hear does not affect the music you are listening to.I knew and I did compare the two. The point is, the ATI's THD is already below the threshold of audibility. You need another reason to explain why you heard such an obvious difference, or it could just be Placebo, but most likely its the way to set up the comparison, that may not be an apples to apples one.
If on your room calibration software you cut off all frequencies above 17K, It will sound different than if you let it go all the way to 20k. This is despite the fact that you can not hear anything above 17K. It is because those frequencies do affect the total sound. Hence noises at levels you do not hear do effect the frequencies and music you are listening to.