I've no doubt you are hearing real differences in this situation. My statement of "always" was definitely incorrect. As I often say,
"absolutes are almost never true" .
"I don’t know about that. I don’t have a Denon but a Marantz with MultiXT32 and playing music though Audyssey, using the pre-outs into my Electrocompaniet in HT mode sounds remarkably worse than playing the same music but uncorrected from my DAC/streamer into the same amp."
Bolded is the reason. Playing with and without Audyssey will make a real difference that will hold under blind conditions. Very doubtful it has anything to do with the DAC. The fact that that you prefer the uncorrected response has everything to do with Audyssey messing up the sound, and nothing to do with the external DAC giving you more transparency. This is quite common with Audyssey in my experience. Unless you do some serious tweaking, Audyssey often does more harm than good.
I think one of the great points you made in your post was that you measured both situations and saw that there was a real measurable difference. I'm mostly speaking of situations where there is no measurable difference, yet people still "hear" differences.
BTW, I was talking about amps in the post you quoted, not DACs, though DACs are even less likely to make an audible difference(imo).
My assertion is not that there will be no difference in sound quality, but rather that there will be no real audible difference in sound quality that would hold under blind conditions. The human brain will ensure that one "hears" a difference, but that difference is unlikely to be an actual acoustical difference. Now, I do think some actual difference may exist in certain circumstances, especially with very difficult loads. However, unless there is a serious problem with one of the units(or additional processing), or one of the units simply can't handle the load, the difference (imo) will be very small. By small, I mean probably smaller than the difference between the left and right speaker of the same model. No doubt my right and left speakers sound slightly different, but I don't stress over it, and I don't buy multiple models of the same speaker just to get better sample. My view on amps is similar, only the difference is even smaller there.
This is all excluding the situation where one needs additional power, or one has an expensive Wilson with a 1 ohm load
. There are certain situations like that where upgrading to a better amp makes real sense imo. Personally, I almost always go for an external amp, but for different reasons. I don't expect to hear any real differences at the levels I listen, but I like the idea of having a good amp that will last me many years and through many AVR cycles. I like the idea of keeping the processing and amplification separate for that reason. On a tight budget though, I wouldn't spend part of that budget on an external amp or DAC. I'd probably get the cheapest AVR with pre-outs I could get, with the intention of buying the external amp later.
I've never done a blind DAC test, but I tend to think most DACs are even more "perfect" than amps, and even less likely to make any sort of audible difference
. I think DACs should really only be bought based on feature set/connection needs. Human brain does the same thing with DACs, though, so "real" differences will undoubtedly be heard under sighted conditions
.
As always, this is just my opinion so far. My mind can change with new evidence.