How can it be that some old recordings sound sublime?

Modern live recordings aside, you're talking about two very different things.  Once you get very far beyond two mikes and a live recording you're putting together pieces of a puzzle rather than taking a snapshot of an actual event.  Non-live with multiple overdubs makes for a tricky process that is not likely to result in a natural sound.  There's also a difference between recording/reproducing acoustic versus electric sounds.  I, too, am often surprised if not astonished at the great sound of some recordings from the fifties, mono and stereo alike.