I've just recieved a new record-player with a variable speed adjustment, and it is like rediscovering music. It is almost absurd to think, but most 78s spins not 78 rounds per minute, and I now know that most Jimmie Rodgers transfers are totally off, he didn't sing in such a high pitch..
But to adjust it, is somewhat subjective, since there is a difference not only by label, but also much depending on the various studios and dates, and then there are plenty things that could play in on what speed the recording were recorded in!