I was there in the sixties and seventies, I saw quad arrive, struggle and commercially speaking, fail.
If there was a good cross section of interesting titles available in discrete four channel on 1/4” or 1/2” tape at the time that I would have purchased, I would have bought a four channel tape recorder~reproducer, but there was so little music in that format to find.
Even if they had made ALL music available in discrete quad, and at a fair and reasonable uptick in price, I would have bought them, but let’s be honest with ourselves, whom ever had to make the marketing decision to commit to producing and distributing, stocking and selling these titles would’ve been voted against early on in house as being unprofitable to the extreme.
Look at this site:
Music on tapes - RecordingTheMasters , and read down the list of currently available high end open reel tape makers offering good commercially viable titles, in the format that many audiophiles believe is the pinnacle of audio perfection, even today, and yet there is conspicuously NO quad of any type.
Check out this link:
Music on tapes - RecordingTheMasters ... and imagine if they had done this in true discrete quad on tape! YIKES!!!
I own five open reel machines that record and play back in four channels at 7.5 & 15 & 30 IPS, but I have to make my own quad tapes from live performance musicians or transferred from other sources because so few titles can be bought NOS, or used, or newly made, it is quite frustrating.