The frequencies in question are high enough they're not generated as direct frequencies by instruments or the human voice. What you find in this region tends to be harmonics from those higher frequencies.
Keep in mind most live sound mixers have their 'high' control around 12KHz - a full 4 KHz below this point where frequencies are being cut off. Which, BTW, means you need to turn that HF knob to +3dB just to remain at unity if you're using a Bose L1 rig.
(it's down because of the physics of the array).
As for the A&H mixer - I wouldn't expect it would have a negative impact on the sound unless it was either set up incorrectly, or your gain structures were just really bad.
I have seen people run a clipping signal into a mixer because they're trying to compensate for a weak PA system or bad part of the PA setup elsewhere. If you are getting a bad sound I'd review the settings and gain structure before doing anything else.
I even wear these when I drive around in my convertible. 
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