You are talking about first gen masterings to CD which were often crap. I'm talking about double and triple dipping by the record labels where they RE-master from the original studio tapes and have FINALLY got it right. Just because a recording is on CD does not mean it must be over-compressed.

The record labels are pursuing this route because they can't sell their new artists to anyone who is not brainwashed by MTV, so in order to keep milking that market segment they keep releasing the same albums again and again. Except for artists like Jimi Hendrix who are constantly recording new tracks from beyond the grave, the only thing they can offer to convince people to rebuy CDs that they already have is with improved sound quality.

For example, just about the entire Rolling Stones catalog has been remastered (for SACD, but the same masters were used to make the CD layers on the same discs that only golden ears with solid-silver cables can distinguish from the DSD versions). Similarly most of Yes, Rush, Sabbath (Ozzy Years), Steely Dan, AC/DC, CSN(Y), Simon & Garfunkel, Dylan, etc name almost any great group from before the 80s and their stuff is getting remastered the right way. Even the Tull catalog has received a make-over, yet somehow they still can't get Aqualung right - at least not mainstream CD releases - DCC did a good job, just about indistinguishable from their vinyl edition.

So, for those albums, the difference is no longer equivalent to a poor-quality MP3 vs a CD, it is more like a high-quality MP3 - say 220Kbps VBR Joint-Stereo and the CD it was ripped from. Which is to stay, indistinguishable for 99% of the population.

And to take it one step further - there are plenty of crap-quality vinyl pressings - 'fixing' them is how boutiques like DCC and MoFi got started. Unless you've got an audiophile label pressing or an expectionally well engineered mainstream release, the correctly (re)mastered CD versions won't just equal it, it will greatly surpass it. Super-22Khz harmonics just don't mean jack when the base frequencies are lo-fi - and for those frequencies a properly mastered CD does not just approximate a waveform it exactly reproduces it. Nyquist's theorem is for all practical purposes an indisputable law of nature, just like the laws of gravity and electromagnetism.