You might take interest in this recent post by David Eduardo
I know that AM receivers in cars for the past 15-20 years have pretty much $ucked at high-frequency response, probably because it might have been the case there was too much adjacent-channel interference.
I have some older receivers at home that, in earlier days, sounded pretty darned good when tuned into AM. And they were still probably using analog phone lines for their STL links.
Gee, I wonder if they'll start rolling off the bandwidth of FM receivers, what with all the pirates and translators and LPFMs crowding the dial. And the FM capture effect being what it is, FM reception is only going to deteriorate.
Some AM stations in the 1960s and even into the early 70s - 630 WPRO in Prov., comes to mind - had clear, distortion-free, reasonable high-end (circa 8 KHz) audio. I suspect 'WMEX and WBZ did as well.
Sad.