Given your question is "Fosi vs NCore/Purifi: When would there be an audible difference?", I think most of us can't be certain that you would hear a difference.
Thanks for the rather helpful reply. I think my question might have been better stated. There is certainly a region where some people are going to hear a difference between two amplifiers and others won't. There is also a region in which you wouldn't
reasonably expect anyone* to hear differences, where the differences are simply expected to be beyond the bounds of the human ear and brain to discern differences (and I'm sure some people would argue here but let's assume we're basing this on published science). (*Yes, all humans are different. Assume then that we're talking about humans that fall within two or three standard deviations from normal.)
And of course this will be use case dependent. How loud do you want to listen, how sensitive are your speakers, do your speakers present a challenging load, what is your room like, how much ambient noise is there in your listening space, etc., etc.. There are some situations in which even a golden-eared engineer might be hard pressed to tell the difference between a Lepai 2020 and a Purifi while in other circumstances the differences would be stark. So my question might have been better stated (but probably still imperfectly) as: Under what circumstances would differences between these Fosi amps and an NCore/Purifi amplifier be expected to be above or below the threshold of what is generally accepted in the scientific literature as being below the threshold of human perception (again, humans within 2-3 SD). The answer could even be as simple as, "There are a lot of normal situations where there'd be a difference," or "there aren't many situations where there'd be a difference." I know this is still not an easy question to answer. We can take as many objective measurements as we like but in the end we as listeners always have a subjective experience. Making a little progress, where possible, towards correlating measurements with likely subjective experience is helpful to some of us