What's a reasonable listening position response curve?

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:

Maximum is maximum and it makes no different if the control has 10, 11 or even 12 positions.