What's a reasonable listening position response curve?

Yes, that to me wouldn’t be very listenable, to hot in the high frequencies. I prefer a curve like Katz described, fairly flat to 1khz then a slope to 20khz very similar to the Harman curve without the bass boost. Of course it's also speaker dependent above 500hz it's mainly the speaker in control , you need a good speaker that measures flat in an anechoic chamber. 

Too true. I was looking more at the ripples of the overall frequency response as a way to understand acceptable variations.

I don't understand the statement that sound quality is 110% determined by frequency response.

That's impossible.

Even if we accept 110% is used as an alternative to "absolutely 100%" the statement is wrong as sound quality is far more than just frequency response.

I also don't agree with the statement maximum tolerable variance is at most +/- 0.05dB, and I would go .01 just to be sure. Might be good with amplifiers, but not for speakers and certainly not for listening position.

From our favourite protagonist/antagonist? Wind him up and he nuts you.

Most amplifiers go to 10. So when they're playing and they need a little extra, where do they go? Nowhere. That's why ours go to 11.

If this doesn't clear it up for you remember the immortal words of Bruce Dickenson: