In the case of Futurama, the only thing that differes between the original episodes and the ones after the resurrection is the music. It is widely known that the full orchestra used in the latter has been replaced by synthesized instruments. This is not a serious flaw in any way, it just smacks of a blatantly lower budget for the series.
Anyway, the wee point I would like to make today is that John K. used a fairly large library of old music that he used in Ren & Stimpy. There are two reasons for this, firstly, John’s love of old music/culture (note the stylized designs and fictitious commericals for powdered toast) and secondly it was a huge cost-saver.
The use of such music does not in any way detract from an otherwise superb show, but it is clear that the two go hand in hand. Listening to the music on its own pulls you back in time to an age of big bands, Hollywood in its prime, the wonder that is outer space and of course, the hustle and bustle of city life.
I’m not exactly sure what proportion of a shows budget goes towards music (if anybody knows, please enlighten me in the comments) and I’m pretty sure it varies from show to show and network to network. However, Ren & Stimpy continue to stand alone in their use of old music. I think it any show (or film for that matter) set back in the day should use old music. But perhaps a more elaborate analysis is needed, which will have to be another day.