The grain of the AM voice

I'm listening to a lot of "old-fashioned" Christmas Carols--by Bing Crosby, Frank Sinatra, Nat "King" Cole, Andy Williams, etc. Harry Connick Jr. also came up in the playlist. I noticed that they all have a similar quality to their voice, which, for lack of a better word, I'm going to call "velvety." I got to wondering about the relationship between this vocal quality and early methods of musical dissemination--AM radio and early records. Were these voices privileged because they sounded good on AM radio (they do, I think--better than on FM)? Were they popular because they recorded well on the early recording technology? A bit of both? None of the above?

Thoughts would be welcome. I'd be surprised if someone hasn't looked into this in some way.

Happy holidays!

I like to watch them squirm...

Happy Holidays!