Great topic, probably discussed here in detail before and perhaps again surprising soon.
Hits are mixed and engineered to fit the era, audience and delivery method, from source to audience ear.
If you are interested in sound and mixes of a hit from early eras you should consider:
What version?
a. Original recorded version (on the medium widely heard by the audience that made it a hit), or something else such as catering to today's audience, even if some of the oldsters think it does not sound right.
b. Mono single version, could be more than one mono mix out there.
c. Stereo album version.
d. Re-recording or re-mix by original artist or re-recorded with a replacement lead vocalist in the group.
e. Subsequent re-mixes and remastering.
f. Regional preference for one particular version.
Cliche, well-known example-
Motown 60's singles were designed to win in a mono sound comparison on the radio, record player and jukebox. The Internet says that during mix comparison shootouts Berry Gordy did not listen to tapes, he played acetates on a record player.
Compared to the all-important single version, album and later compilation stereo versions frequently sounded lame. Many who heard Motown hits on stereo FM have never heard the powerful and kicking sound of the original mono version. On the singles, engineering talent specific processing could have been added in the disc mastering stage, that sound is not on the studio multi-track work tape or the studio mixdown. For the most part, the mastering guys are gone or not talking. In the Motown arena Hitsville USA compilations are pretty well sourced with original mono versions. Other compilations are simply horrendous, and they have been heard on FM stations by millions.
Here's an exercise- pick one 60s hit and listen to every single version found on You Tube. Then go to the other pay streamers and see what they have. If you have them, listen to original vinyl or CD version. Now... determine what you would play?
Note- You Tube poster WABCRADIO77 runs everything through a single band limiter. He has the best sourcing of original hits, but you have to remember he has a limiter on it.
My view:
I think if a radio person is going to play old music, recording and mix curation is important. However, time and money involved in this must be work within the business plan. Truth is, most of the audience does not know what it really sounded like, but they do know how it makes them feel.
Radio people are different, I am annoyed if I don't hear what I believe is the correct version. For example, if I were to play "Rainy Days and Mondays" by the Carpenters. I would use the version without the replacement ending present on later versions. No disrespect to Richard Carpenter, he is blessed with awesome talent, but they nailed it on the single.