For some more reference:
1. This seems to be noticeable only where the amp is most power hungry. I can't hear this change in pitch at high frequencies, and I don't hear it much up in the upper registers of the woofer's range or tweeter's range at all, and I don't hear it at all in my subwoofer's range. I am now using 200hz as my reference for this test.
2. For what it's worth, my soundcard is set at the -10db output setting and I have an additional 7 db of attenuation going on at the mix level. The source signal strength is at +0db.
3. With that frame of reference, I hear the drop in frequency response starting at volume level 12 on the receiver. By 15, it's about 1/12 of an octave down in pitch. By 19, it's 1/6 of an octave down. By 25, it's 1/4 of an octave down. This is rough and
slightly exaggerated. But not by much. The 200hz reference is throwing me off, at low levels it just sounds out flat because it's so close to 220hz (low A.) I should really use 220, but I'm lazy

4. Yes, I am listening to this and judging the relative pitches by ear. It is clearly obvious (to me, at least.) I've been playing fretless/valveless instruments like the violin and trombone as an amateur for about 20 years now, and I am currently studying music history. We can't pass our music theory classes if we can't hear this stuff

5. At 25, it's really pushing it. I'm getting a headache and I hear some distortion. I'm a bit dizzy and my ears ring a bit. Since it is +0db, I believe this is the loudest the amp can play at this level, meaning a dynamic "peak" but being sustained by the signal generator.