Yes, this is the crux of the problem. I have ridiculously sensitive ears, and the times I've heard MP3 audio I've been disappointed with the quality.
Let's face it... most music these days is not "pure"... they've all been tinkered with during the recording to CD transfer process... especially older albums, which have been remastered.
Naturally there's nothing pure about most music these days given all the processing that happens during recording and mastering. But I'm not interested in absolute universal purity. What I'm interested in is hearing a pure reproduction of the master recording with all its impurities.
That may sound strange, but really not, because not all impurities are created equal. The impurities added during recording and mastering were put there by audio guys to make the music sound better. MP3 and other compression schemes, on the other hand, were designed by computer guys to decrease bandwidth. The one has to do with creating music, the other with saving hardrive space. My ears tend to appreciate the first but not the second.
I have an old Jazz album that still retains the scratchy recording sound. The only thing "authentic" about the experience is that it replicates the inferior recording technology of the 20s/30s. The person hearing it live back then didn't hear those scratchy noises, and I doubt the artist intended those noises to be present either.
Well, I think a scratchy old jazz recording is an exceptional case. What bothers me is hearing high-quality modern recordings that are missing dynamic range and high-frequencies due to MP3 compression.
Like I said, I'm picky :)