I've never run across this myself, but I was reading in another forum about a person who bought a FLAC download of a classical album by BIS that sounded like it had a frequency boost in the treble and upper mids. He ordered the CD and it played fine in his player, but when he tried to rip it, his ripping software indicated that the CD had been encoded with pre-emphasis. He ripped it with and without correction and it turns out that the rip without correction sounded exactly the same as the sizzley download. Apparently some ripping software handles pre-emphasis perfectly transparently (iTunes) and others make it an option that is easily overlooked. Pre-emphasis isn't common, but it's something to look out for when ripping discs or buying downloads.