That is merely a rule of thumb. In practice there is a massive spread depending on how much use the unit has actually seen, what environment it was in and what capacitor quality was like to begin with, plus some sheer pot luck. Parts in always-on circuitry tend to age a lot more that those turned off as well, so units with soft power from the '90s can be more problematic than their older cousins with hard mains-side power switches. (The PRA-S10 belongs to the latter variety, thankfully.)
Possibly a compromise between CMRR and noise, not a very good one mind you. The input unsurprisingly is not AES48 compliant either, although the presence of a 10µ cap between XLR shell and signal grounds indicates an attempt to deal with pin 1 issues. Looks more like high-level bodgery to me than them actually knowing what they're doing. (It is also old-fashioned in that it seems to be pin 3 hot, while pin 2 hot is the accepted standard these days. Pin 3 hot is uncommon post-1990 or so.) If it was my unit, I would want to modify the line receiver resistor values for proper balance and -6 dB at the very least.
Side note, seeing a discrete folded cascode input that actually seems to work well on the main amp is a bit of a surprise. These can be really good in IC form but discrete implementations are often disappointing. Perhaps the choice of a dual JFET for the input is helping matters. Mind you, this amp sees so little common-mode action that it may just as well have used a traditional cascode.
I'd be whacking on some 220µ-470µ/10-16 V caps in parallel to 2.0 V zeners ZD201/202, that should quieten down the current sources (which in turn would hopefully address some of the elevated noise seen when turning down the volume, assuming it isn't actually coming in via the supply rails). Usually when people need low-noise voltage references in this region, an LED would be the more common choice these days, but this trick may not have been very well-known pre-2000s.
Well, if you would be willing to send the unit off to Germany, yours truly has a Levear VP-8193D and some experience measuring tuners with it... (Side note, isn't FM broadcasting gone in Switzerland now? The Säntis frequencies used to carry far inland and I was kind of sad to see them gone from the band here when I revisited FM earlier this year.)
