How can it be that some old recordings sound sublime?

Agree with all of the above. Great tube equipment with people who cared about the sound. There is something very transparent about many of these recordings both classical and jazz. Like the Nelson riddle Capitol recordings from the late 50s. Fritz Reiner bartok recording from the mid fifties. But there are also great recordings being made now. The most recent Justin beiber record "purpose" is very beautiful sounding and I have a several great orchestral recordings of current composers music that sound transparent and beautiful. 

The point of the whole thing was different, too.  Getting a good reproduction of what was being played used to be a priority.  Getting the listeners' attention in a crowd seems to be more important now.  Different equipment, different approach, different priorities.

Say, wasn't that prior to the introduction of digital? No offense intended.  Lol

^^^ Its been said that digital bits completed the job started by transistors ... the ruination of reproduced music in the home.

Some of my most enjoyable (and best sounding) recording were never release in stereo. Good examples are Miles Davis' "Round Midnight,"and Brubeck's "Jazz Impressions of The USA."  Glorious sound and performances.  

Yes there are definitely very  good mono recordings. Sarah Vaughan with Clifford Brown is a prime example and one of my favorites.
Charles,