So I am using a Marantz CD-400 (no B) which I am told is the same as a JVC 4DD-5, but with an added "aux" input. This is playing into the CD-4 input of a Marantz 4270. The 4270 also has the SQA-2B option board installed. (yes, you need both the CD-400 and the SQA-2B if you want all three formats. "Vario-Matrix" aka RM aka QS is built in.
This got real weird trying to figure everything out. Using this system is not intuitive. So I have my turntable hooked to the "Phono" input of the CD-400 and the "4 CH OUT" front and rear RCAs connected to the "CD-4/AUX" line in RCAs on the 4270. That part works as you'd expect. Here's where it gets weird though. If you want a 2 channel or matrix LP to play only through the front speakers, you need to also connect an interconnect from "2 CH DIRECT OUT" of the CD-400 to the "PHONO" input of the 4270. Then to play a 2-channel, SQ, or QS LP you reach behind the CD-400 and flip 2 CH DIRECT OUT switch to ON and set the receiver to PHONO. Otherwise, it'll play through both front and rear speakers, and who knows what'll happen if you try to apply SQ or QS decoding on top of that. Then if you want to go back to playing a CD-4 LP you need to flip the switch back and use the CD-4/AUX input. Yep, I really wish they'd arranged the switches on the CD-400 differently, but that's the way it works.
If you are using a Sansui QRX-7001 you need no demodulator, it will decode all three formats natively. I'll be hooking mine up tomorrow to see if it works as I'm pretty much done shaking down the Marantz setup! (I know it works, but I have not tested CD-4 playback as I did not have the capability the last time it was hooked up.) That works much more intuitively and by reputation has the best matrix decoders of the era. We'll find out I guess!
In other news, I found another AT-15Sa cartridge and I'm getting CD-4 decoding from my Dual 1229Q (as well as my Technics SL-1650 which was the first one I got working)