Why do my ears ring with my new amplifier?

Wise words Tubegroover .
I find that some speakers do well at low volume, most don't.

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.

"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?"

Some people roll off the high frequencies either directly with some type of EQ or tone controls, or indirectly using system matching to deal with problem frequencies. Altering frequency response, even if its to flatten it, will make the system sound worse (to the person who set it up, not necessarily to everyone.).

"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."

There's so many other factors that can contribute to an amps listening fatigue, besides a flat response, a book could be written on the subject. Timbre, distortion, material components, power output, phase, feedback, are all qualities I don't see as being irrelevant. You can if you want to. That's you're subjective view on the matter. Not everyone would agree with you.

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.

"Yours is also your subjective view.

Of course it is. How could it not be?

"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."

I don't see how you can go from that to this.

"I'm well aware there's more than one kind of fatigue and the one in question is the painful kind."

Maybe you should pick one. Either you have one factor and everything else is irrelevant, or there's several factors that are relevant.

"I would say pain is the most relevant."

Maybe that's your view but another person may say different. Everyone makes their own choices. That said, I think most people would choose pain as the number one cause of listening fatigue, like you. And that's exactly why so many use EQ's and tone controls to roll off the top end. In doing so, they're moving frequency response away from flat on purpose to make the system more listenable.