You know it's funny.... of ALL the demodulators I've had: [ Two Panasonic SE-405's, Three Pioneer QD-240's, a Technics SH-400 and a Marantz CD-400B ] of course, the only one out of the bunch that had automatic gain control on the carrier level.... DIDN'T WORK! A buddy loaned it to me for experimentation purposes but the silly thing had some issue and would never go into CD-4 mode. Made a heck of a nice stereo pre-amp though and that's what he uses it for today.
Still, it makes sense to have auto gain control on the carrier level for the same reason Stereo FM uses the same principal. I've just never had the luxury of experiencing it!
I think that had CD-4 (and Quadraphonic sound in general) persisted a few years longer the new demodulators would have all had automatic gain control of the carrier. Both of my 4-channel receivers have it and both were made in 1975 which is getting to the end of the quad era. Though, both Sansui and Pioneer produced their flagship models until around 1979, I believe. I know that in 1979 when my JCPenney Quad receiver (a very basic unit with only RM and SQ decoding, probably made by Panasonic) fried itself and I went to buy a new receiver to replace it the only units I could find were the Sansui QRX-9001 and the Pioneer QX-949. Both were priced well above $1,000, so I ended up with a Sansui stereo receiver which I used for over 30 years on a daily basis.