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Perhaps longer-term stability was less of a problem with 1980s technology? In the HF world, I have a couple of Sony ICF2010 receivers, and a Liniplex F2 (excellent piece of equipment) that use the phasing method for sideband selection, and none seems to have suffered any deterioration over time in terms of sideband separation.
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There is a world of difference between maintaining accurate P and P+90 over 300-3kHz for communications purposes, and the requirements of a decent stereo receiver. Probably the best method for communications is polyphase, but that would be useless for stereo as P is very frequency dependent. Getting accurate phase and amplitude tracking of three signals at F, 38-F and 38+F is hard! Nowadays I suppose you could do it with a digital filter.
Quadrature modulation would be OK, provided that it was an accurate 90 degrees in both transmitter and receiver in order to avoid crosstalk.