Robsker, the reason why those recordings sound better is because the musicians playing the music were better. It's that simple. In the recording world, it's all about performance over equipment. You want to catch lightning in a bottle, and these old recordings did it.
There was nothing magical about the recording process. In fact, the equipment, by today's standards, was fairly poor. And just going by pure specs it was a nightmare. THD, wow and flutter... these things were off the charts.
I'd rather catch a breathtaking performance on a Tascam cassette 4-track, than a lifeless take through a Lynx AD converter.