You remind of back in the day ordering "music circuits" from British Telecom for broadcast use--this was their designation for audio lines that would be used to carry programme material, either for radio or to accompany video for TV.
BT used simple 2 wire twisted pairs--not balanced, not screened. What they DID do was use line driving amplifiers and insist that the interconnect was carefully impedance matched (600 ohms by memory). Working this way they fed audio for hundreds...and even thousands--of kilometers.
Ah, the old days before fibre. Nowadays broadcasters can feed super high quality anywhere--but use Skype instead.
Ah yes! PO lines. Angus McKenzie, the FM guru of HFNews would often comment that the line from the Liverpool Phil to Bush House had a crackle on it a week last Sunday night!
I think the death knell sounded for copper links when stereo came in. It was tricky enough to EQ a line for a flat response, to get low phase shift meant matched pairs and that came expensive.
Interestingly the digital MW links that replaced them were about 12,1/2 bits iirc but absolutely NOBODY noticed or complained about "clinical" digital sound!
Just thought! Yes, stereo could be sent as a sum and difference signal which eased things a bit but I don't think the lines could be TOO different even then? Remember, there were no 16bit, 9volt delays you could pull out of a pocket!
Dave.