When I wan t to shock people with how good old recordings sound, I almost always pull out jazz examples. Almost any late 50's or 60's "six-eye" Columbia sounds great (my demonstration go to is Ellington's "Blues in Orbit" which is late 50's stereo and better than most current stereo recordings for quality; the subsequent reissues, even is SACD do not sound as good). Another demonstration choice is Sonny Rollins' "Saxophone Colossus" (a GREAT recording for music and sound quality and it is on available in mono).
Old, original release Blue Notes are way too expensive for me to own, but, I have heard them and I like them fore than the premium reissues, although those are still quite good and worth having).