Why do my ears ring with my new amplifier?

I think you're arguing with yourself. Carry on.

The flatter the frequency response, the less fatiguing the amp will be. It's just that simple. Whatever other attributes/detriments it has will be irrelevant. Just because you spent huge money on somebody's 'recipe' doesn't necessarily make it soa. It's been flavor of the month as far as amps go. The best ones are both unremarkable and spectacular at the same time. If you can't say that about yours, then I'd be curious of the shape of it's response on a foot long tape. The best is also the amp you can crank to levels that surprise your ability to tolerate. The others you have to be 'careful' with.

What about the pastor?

I'm not interested in arguing with anybody. But just for argument's sake, how does it not make sense to you that a flat FR will be less fatiguing? The op's confirmation that the high frequencies are not the problem are a testament to this premise. Whether induced by the room or a component is irrelevant. I just happened to focus on the amp since it was his focus also. The fact is you can't add something without taking something away. It's that 'added' part that ends up like a spike being driven through your head which may be what's fooling you at low/moderate levels. Speakers have their own general limitations that smart ass amp designers think they can circumvent with their hair brained ideas on how to reinvent the wheel. "I know, lets put a nice bump in the bottom end to take care of that age old extension problem,if the guy gets a head ache, let him not listen so long". Or,"Hey, I think I'll just boost the mid range a bit, give it some added 'presence', if the guy starts feeling spikes shooting into his head, let him turn it down". The way I described an amp with a truly flat frequency response is correct. Unremarkable with like material and spectacular with like material, both of these extremes exceeding amps with less than flat response, all other things being reasonably equal of course. I find it utterly ridiculous when the tube guys declare 'excessively low distortion'. Laughable.

Yours is also your subjective view. I'm well aware there's more than one kind of fatigue and the one in question is the painful kind. I would say pain is the most relevant.

Well both of us can't be right, right? So you may as well be wrong! WRONG I SAY!! WROOONG! I'm right... yep, right. But you... you're wrong, again.

That's right! And don't you forget it! That'll be 'Mr.' Csontos for you from now on.

Btw, that sure didn't take you long. Is there anything else you do during the day? Just curious.