There is irony there, you are right. But you are in a position where you don’t know what you don’t know, so it’s not really a fair argument. There are some things most people don’t know off of the top of their heads that apply here. First: The frequency response of the amp of the Onkyo is probably (and yes, I’m guessing, based on experience) very close to ruler flat in the range you are using it, especially if you are using subwoofers. The distortion at the levels you listen is probably not audible. The exact same is probably true of the integrated amp you are now using. Even if these things are not true, then the odds are massive that the audible difference between the two is slight, and the odds that there is a huge audible difference, especially at the high frequencies, are extraordinarily low. So the probability of your assertion seems to be extraordinarily low. IMHO. The integrated amp may well measure much much better than the Onkyo receiver, but even so, the probability of your assertion, that there there is a huge audible difference, especially in the treble, between the two, is extraordinarily low, IMHO.
Based on the frequency response measurements of Sound & Vision, assuming those are even in the ballpark, there is room for huge audible improvement in your speakers, at a not very high price point. Having two subwoofers is a giant step toward really good sound, and I would guess you have nice ones, so pop in some better speakers and that is where you get your huge improvement in sound. On the other hand, if you are happy with the sound of your system, there is nothing wrong with that at all. I can even enjoy music on a transistor radio, no biggie.
Speakers do make a big difference. One thing this forum is buiding up to do is help us make good choices in loudspeakers. Most of the audio press is junk reading and nearly all of us have been flying blind in choosing speakers for many many years.
The advantage some people here have is they know what in audio is likely and what is not likely based on experience and book knowledge, and so when someone else makes an assertion that they understand to be exceedingly improbable, expectation bias becomes a very likely explanation. Even so, you see some of even their expectation biases being revealed after equipment tests in a way that contradicts their expectations or subjective impressions. And yes, they can get a little worked up about that. ; ) We’re all human that way.
I truly, sincerely, hope this is helpful.
Back to the smoking llama (I think) and megaphone monkey arguing about. . .filters?