Well, I would hope that anyone making either assertion would qualify it to something a bit more meaningful than 'there is no difference between a master and a lossless file' and define 'marginal gain'. In reality of course, people rarely do, it's much easier to work on assumptions than solid evidence.
'there is no difference between a master and a lossless file'
'there is no difference between a 320kbit MP3 and an uncompressed WAV'
which is provably false in the sense that the files, at least, are different, the MP3 being smaller.
'you cannot tell the difference between a 32kbit MP3 and an uncompressed WAV'
which is also likely false for most, if not all, listeners and samples
'you cannot tell the difference between a 320kbit MP3 encoded with the latest version of LAME and the CD master'
which is true in general, but there might be some tracks that can be distinguished (note, tracks, not listeners, to the best of my knowledge there is no 'golden ear' that has ever been documented that can hear the difference in general)
or it could mean
'there is no audible difference between the DTS Master Audio 5.1 on a blu-ray and the DTS core 5.1 track on the same disc'
which is certainly true for a percentage of tracks that is very close to 100% (we can't say it with certainty that it is exactly equal to 100%, since we have not tested all possible samples, which are of course infinite) as shown by scientific testing http://tech.
'there is no audible difference between the DTS Master Audio 5.1 on a blu-ray and the Dolby track that was on the DVD I upgraded from'
which is probably false, especially as the DVD might not even have been encoded from the same source, but a dangerous claim all the same because almost nobody making these claims is doing so with any kind of scientific method or proper testing.
'Audiophile' for me is a dirty word, because it stands for people making ridiculous claims such as that a movie from the 1930s re-released on blu-ray is deficient because it doesn't have lossless audio, even though the original recording process on antique equipment would have irretrievably destroyed much of the sound signal, and the lossy audio used is of a sufficient bitrate to be indistinguishable from lossless anyway (and why the lossless fetish, when the recording process is lossy, the video is lossily compressed and the original cameras + film used lose much data, your eyesight and ears are not perfect, and so on); and getting people to waste collectively billions of dollars on worthless upgrades that deliver no audible improvement. (Example: blu-ray audio, a completely pointless 'upgrade' over CDs, even though the discs might actually be better, it is not because of the magic extra bits, it is just because they have been mastered better, a mastering that could have been done on CD with equal fidelity.)
I just did a quick post before going away , and of course I forgot a word in my sentence. Had I known that it would send your head into chaos and make your brain swim in various conclusions, I'd never have posted it. Sorry. And where did you see any assertion, I was just trying to put up an example ( incomplete, but you know...) about how audiophiles and non audiophile have the same attitude sometimes. Sorry, my shaky english sometimes makes things weird for me, and I must sound weird too. But my sentence wasn't meant, like you say, to be the proven truth , with proofs. It was just an affirmation, not a complete claim, nor a debated fact supported agressive opinion.
@loveintheHD : Sorry about me answering " agressively" , I've misunderstood your post. Edited it to make it less agressive. And I must add I won't comment any further to do not detract the OP. Again, sorry for the words omission.