As I go over the owner's manuals for my hifi stuff I find most of it to be pretty good. And as someone who wants to know the minutiae of my gear and does a lot of technical writing (geez, look at most of my posts!), I'm fairly critical. I agree the Apt preamp set the standard in '79 - I still have the manual.
Among my current stuff good marks go to Sonic Frontiers, Proceed, Yamaha (for their cd recorder), JVC (tv&vcr), and my '80s Tandberg, Thorens and Phase Linear (for their old 1000 II noise reduction rig). Adequate, but OK for one's needs for Thiel, Headroom, BelCanto, Philips, PS audio. More effort could have gone into the manuals for McCormack, Nakamichi, Magnum-Dynalab.
I don't know if I can draw conclusions about how good the manual is vs company size, age, product, expense, country. I've seen plenty of bad manuals, it seems it's just a matter of priorities between marketing, the designer(s) and company philosophy as much as anything...