Well there is aiming for a market that requires differnt production- such as FM radio. I would say the majortiy of the older records that dont sound good (Led Zepplin comes to mind) are usually about the gear used and the limits of mastering and storage at the time. Gear used: I love Genesis, but most of the records have no bass- which usually means the studio monitors/mastering studio had too much bass. Some records have poor dynamics, could be because the tape machine in use had no dynamics due to the way it was made, the head quality or the tape. Some even older records have no high end- generwally because the gear used in the recording didn't have any or maybe the monitors in mastering had too much top end.
In general the breakthroughs in quality started happening around the 80s. Earth Wind and Fire, a real breakthough as almost all R+B was motown sound prior to that point. George Massenburg recorded that with his own handmade GML gear, and knew how to get more bass on a record. Very skilled. Sheffield Labs recordings (Doug Sax, Bill Schnee) were a breakthough and tose recordings wqere amazing. Fleetwood Mac (Ken Calais and RIchard Daschutt), wow, still sounds good. But listen to some other records of the same time frame and they are messy things with many flaws. FInd the best engineers to find the best sounding records, do not follow the artists- they are on the other side of the glass.
