[germain]

You are attempting to compare the file source with the digital storage/transfer. Two totally different things. Prefm refers to reels/cassettes mixed and recorded at the show for later fm broadcast. When these tapes are transferred into the digital realm, they could be stored as lossless files like wav or flac, but someone might also store them as lossy files like mp3. Obviously you'd want the lossless copy over the mp3.

Numerous factors play into the quality of any given analog source. For your prefm sources, all of these factors will make a huge difference in quality.
The mix going into the tape and recording levels of the tape.
The type of recorder & tape used.
For a reel, the speed of the recording and whether it was recorded 1/2 track or 1/4 track.
The number of analog generations before the tape was digitized.
Tape degradation before it was digitized.

Then of course the analog>digital transfer can make another huge difference in quality of the final product.