To me, there seems to be three basic sound quality disussions on this board.
1. SOUND QUALITY COMPARISON BETWEEN DIFFERENT ERAS. When searching this forum for information on certain artists or tracks, for me it's not that important to get information about sound quality of a 1934 track compared to a 1955 track since the 1955 track will win almost all those comparisons regardless of reissue company. The importance is getting info on the specific 1934 track on one reissue compared to the same 1934 track on another. I can realise myself that most tracks recorded in 1925 will not sound as good most tracks recorded in 1940, which in turn will not sound as good as most tracks recorded in 1955. This discussion can therefore be left fairly quickly. It's like comparing apples and oranges.
2. SOUND QUALITY COMPARISON BETWEEN DIFFERENT RECORDINGS OF THE SAME ERA. To, me this will be a very difficult discussion since for example comparing a good sound quality transfer of a recording you don't like probably will lose out to a bad sound quality transfer of a recording you love. There are simply too many variables. It's like comparing apples and oranges, just in a different way than when comparing between eras.
3. SOUND QUALITY COMPARISONS BETWEEN DIFFERENT TRANSFERS OF THE SAME RECORDING. To me, this is the most relevant sound quality discussion. But is has to be done with comparisons to make sense.
Most often, I think talking about sound quality when it comes to 1920's-1940's recordings without comparing between diffent sources ("original" 78, "reissue" 78, reissue LP, reissue CD from Hep, reissue CD from GRP, reissue CD from Proper, etc), is sometimes close to pointless, since we are very different in our opinions about what is poor sound quality and what is good.
One problem with talking about sound quality without to making comparisons can be that if you like and dj mostly vintage music, you're probably bound to find more 1930's recordings having ok sound quality than if you like and dj mostly later jazz (from 50's and onwards), which will give you different answers on the sound quality issue if no comparisons are made between different reissues of the same material. This is probably then an unaware comparison between eras.
Though, even if you have two different reissues to compare (for example the same songs on a Proper release and a GRP/Decca release), tastes can differ on what is important in sound quality.
For example, while I find that most GRP releases have more clarity and life than Proper, which I find muffled and dull, thus giving the "sound quality" crown to GRP, someone else might find the GRP release too light on the bass and with too much surface noise, and the Proper having more bass and less surface noise, thus giving the "sound quality" crown to Proper.
This comparison between different transfers of the same recordings will still be the most important to me.
Then also, it seems that over time everyone will find the reissue labels that are the closest to their liking, as I enjoy GRP/Decca, others enjoy Proper, even others Classics, etc. And in the end, often we don't have a choice with older recordings, there migt be only one reissue out on the market, so that I'll have to settle with Proper, when GRP, Hep, Masters of Jazz, Mosaic, etc haven't reissued the songs I'm looking for, or are out of print. Enter second hand...
Well, I've ramled on long enough, I'll escape while I still can find my way out of my own web of words. Damn english language...
/Jonas