I set the turntable (Pro-Ject Debut Carbon) up when I bought it according to the manufacturer's guidance, but given it's been a few years, I'll be replacing the cartridge, and the necessity for precision when using CD-4, I'll do it from scratch when I get a new cartridge - there were no protractors involved first time round!
Some are sealed, but they're mostly pristine. I'll dust them down, or wet-clean them if necessary.
I'm in the UK so CD-4 is mostly imports. I'll maybe focus on Japanese in future if this project works out.
Sadly the set-up disc was missing, but they're easily available second-hand so I should be able to fine a good one. I'll check the forums for reliable titles to get me started, but I have to say it was a surprise to discover that CD-4 doesn't just work out of the box and needs to be tuned. I'd presumed that given it is discrete, there would be full separation rather than a trade-off between that and sound quality.
Looks like I'll have to keep my eyes open for old stock AT440MLs unless someone reviews the replacements. As a beginner it's probably unwise to procure legacy stylii as I'm not sure I'd be able to identify degradation as the weak link in a completely untested setup.
I've checked online and all Linn interconnects are low capacitance.
I checked out the Behringer UMC-404HD and it does support 4 x 24/192. It's around £100 new so hopefully I'll be able to pick one up even cheaper second-hand.
Unfortunately my receiver doesn't support multichannel analog input so unless I replace it I'll never know the pleasure of putting on a quadraphonic LP, but I've got a few records that haven't been made available on the quad scene, so I'm really looking forward to hearing some of those long-lost mixes.
I've just been reading about the Stereo Lab softward decoder and it looks really interesting, and I'll probably give it a go in the future, but it cost around the same as a second-hand audio interface, so I'll go down the analog demodulation path for now.