Regarding CD reissues, check out the Music thread 'HDCD List'; a number of reissues in the past used Pacific Microsonics AD converters during the remastering process, and it's been fairly accepted that those converters sounded good. You'll find a good number of titles there in the thread, and while there is the matter on what classifies as a 'true' HDCD (you can read all about it starting around page 35 on), it's reasonable to say the engineers probably won't have used those Pacific Microsonics boxes if they didn't sound better than what was available at the time. (It seems HDCDs were more available - labeled or otherwise - around the mid-late 1990s to the mid 2000s).
Also, if you're interested in sound quality in the physical form (at least theoretically - of course it all depends on the mastering, right?), you could check out reissues in SACD and Blu-Ray Audio. For example, I have a Blu-Ray Audio disc (24/96 LPCM and the latest hi-def Dolby & DTS formats) of Ella and Satchmo and it sounds great! My hybrid SACD copy of the Kleiber Beethoven #5&7 has the beauty of surround sound, and IMO both stereo and surround SACD layers sound better than the CD. Finally, the landmark Miles Davis' Kind of Blue as been reissued in several iterations.
