Historic recordings- best audio on reissues

Anyone else a fan of historic recordings?  There are some great reissues of classical and jazz performances (and blues) going back to the 30s and earlier. But many of these performances have been reissued more than once on CD. This is partly because it's easy- a lot of these are in the public domain. But often genuine effort is put into improving the audio. Which one sounds best? 

For example,

- A recent reissue of Benny Goodman's famous 1938 Carnegie Hall concert sounds dramatically better than the original Columbia CD because some guy spent a year and a half cleaning it up (which, in turn, sounds better than any LP because a better source was found). 

- There are two releases that I know of for Furtwaengler's 1954 Beethoven's 9th, one a bit better sounding than the other. But there is a CD of a 1955 performance that is just as good and sounds much better.
- There is a "CEDAR" release of Mengelberg's 1940 Brahms German Requiem. How much better is it than the original Philips CD release?

Amazon reviews are not always available for these recordings, this too early for Steve Hoffman's forum, and other sites seem extremely hit-or-miss.  Where do you go to get information? I can't afford to buy all of them and compare!