I can't imagine stereo done with one channel on FM and another on AM could have sounded anything but horrible. Unequal frequency response and noise for two things, but more seriously the phasing of the speakers of the two sets would randomly be in phase or in anti-phase. Depending on the speaker connections, the number of inversions in the respective audio amps, the phasing of the AM detector and how the antenna happens to capture the AM signal, and if the IF strip flips the phase or not.
Ever get the speakers on a stereo system out of phase? Gives a strange "pulled apart" effect on the ears.....
