I have a HF loss that starts at 2.5 KHz, as well as tinnitus (the Army didn't issue me ear protectors in basic training 53 years ago), but I could easily hear the difference in HF clarity when I changed from my Adcom TPA500 to the 990. I'm skeptical of any sweeping claim that the absence of signal above 20 KHZ is undetectable by humans, especially if they have a chance to compare it with similar material that has ultrasonic signal content. I think there would be a difference in subharmonics or beat tones down in the directly audible range. I'd like a chance to blind-compare the sound of my Maggie 1.6QR's (- 3db at 24 KHz) with the 3.6R's, which go up amost another octave: I would be surprised not to hear a difference.