There is a world of difference between exchanging arguments and quarreling. Please everybody, cool down and keep to the former. And please guard against interpreting arguments as insults and answering "accordingly" with irony, sharp words and so on. It would be a pity to turn this thread into a fight and, in my opinion, it would be disrespectful to the OP.
This couple of statements is a good example of opinion I disagree with. But for me, disagreeing with them does not make me believe that Barry would be naive, not knowledgeable etc. I just happen to think differently and I'm prepared to explain my reasons.
So, let's just imagine that you are upgrading your gear: selling your old dac, amp or whatever and buying a new, presumably "better" (and often more expensive) one. What is your expectation? To hear an improvement. Perhaps not immediately, but over a little time, say over a few hours (as after a longer period of time the memory of the "old sound" will recede and be partially contaminated by the new impressions), you expect to hear a "better" sound. To be struck by the discovery that "those cymbals are sweeter than I remember", "now I can finally understand what the guys in the background are saying", "the bass foundations of the music seem stronger, more solid now" and so on and so on - and above all, a generally feeling that you are now enjoying your music more that you previously did. This is your objective when you are upgrading. (In most of the cases, you will also notice some things missing - the sound used to be warmer, for example - as there is rarely a piece of gear superior to another on all the subjective parameters unless it is in a totally different class, and often much more expensive too). And how do you make the comparison between the two dacs? Definitely not by fast A-B switching, but rather by being impressed by the new sensorial input over a short period of time as opposed to the memory of the old "sound", memories created over months or years of use.
By using fast A-B switching as a comparison methodology we presume that whatever we can hear on the longer run (minutes, hours or more), as a sum of often each on its own unconscious impressions, we can also hear through fast switching. This presumption has not been proven afaik and some may find it doubtful. I have compared the two CD players in my house, And by fast (not instant, but by swapping cables) switching I could hear something was slightly different (they were much more similar than I would have expected!), but I could not quite put my finger on it. Only after listening to a few minutes in a row on each of them, and going back and forth for a few times, I could articulate my findings, and the differences became subjectively more obvious..
Now let me address a few obvious objections to the validity of the conclusions of the audiophile above:
1. There is no level matching whatsoever.
Well, in a way there is. Over time, I will listen at various sound pressure levels, both on the old dac as well as on the new.one. If I can hear the already identified differences both at high as well as at low volume (for example there is a greater sense of space, or transients seem faster and snappier), the lack of level matching becomes unimportant.
2. Expecting to hear a difference makes you hear it when there is none. We are too suggestible.
This is a strong argument. We are suggestible indeed. But this can be mitigated by training, and some of us are experienced audiophiles with perhaps tens of years of listening to audio gear. Also, sometimes we expect to prefer the dac A because it's more expensive and "should" sound "better", only to discover to our great surprise that, while the two dacs in question do sound indeed different, it is dac B the one that we prefer. Such findings that go against the expectations (and wishes!) cannot be explained through suggestion. Placebo is an easy explanation, so it's rather overused. It should be just a suspicion, something to guard against, not a certain explanation unless directly proved such is the case.
3. The memory is unreliable.
Up to a point. But fast A-B switching does not take it out of the equation. When I'm listening to B, the A sound is already in my memory and that's where I'm taking it from to compare against B. Fast switching means I have little time to forget details of A, but also means 1) that A is just a short, fast impression, perhaps making it more superficial (for example I'm not used to listening to gear with very good imaging capabilities, my hearing is not "trained" enough in this respect and I might miss just how good or bad the imaging is during such a short exposure to sound), and 2) that the B's proximity in time to A might make it more prone to be influenced by B, perhaps by tiring the ear or by some neurophysiological mechanisms on the cerebral cortex.
4. Whatever differences there might be that cannot be reliably detected by fast switching are too subtle to matter.
That's a very subjective statement! Your "too subtle" might be my "annoying" lack of instruments placement precision, for example, or vice-versa.