Recording quality by decade

As I've been listening to my records, a pattern seemed to emerge that prompted this question - did the recording technology advance significantly between the previous decades and the mid/late '70s? Most of the classic rock records I own pressed in the '60s sound like crap compared to the classic rock records recorded in the mid to late '70s.

My Cream, Doors, Led Zeppelin, Beatles, and Jimi Hendrix records, just to mention the biggest acts, sound awful compared to Pink Floyd, Foreigner, Supertramp, Kate Bush, Rickie Lee Jones, or Fleetwood Mac records I have that were released in the '70s (and '80s). There are arguably a few exceptions, such as good pressings of some of the Led Zeppelin records, but on average any record recorded and pressed in the '60s sounds just bad compared to most records from the '70s and '80s. All of the Cream records I have are just painful to listen to - muddled, veiled, flat, and essentially garage quality.

I understand I'm making a big generalization, but seriously, I can't think of one record from the '60s that sounds really good. This puzzles me as there is a plethora of superbly recorded jazz records from not only the '60s, but also the '50s. Has anyone else noticed this?