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Now, this digital information bothers high fidelity tuners. they are coming out with modifications to the
ones with the worst susceptibility. I mean even tuners built in the 1970s. Some of them were damn good. Now,
the digital information makes for like "hash" or whatever, an unpleasant background noise.

I saw a modification for it and surprising to me is it was not something to cut the IF bandwidth, it was
strictly in the ratio detector. It seems the newer quadrature detectors don't have much of a problem with
this, so far I saw.

The digital noise is commonly a problem as the result of the way
stereo demultiplexing works. The higher-frequency components of the
FM detector's output are mixed against the 38 kHz local oscillator,
thus doing an AM demodulation of the L-R subcarrier band which lies on
either side of 38 kHz.

Many MPX decoders use a simple switching-mixer architecture, and these
will end up detecting both the signal around 38 kHz, but also whatever
is present in the neighborhood of the third and fifth harmonic of 38
kHz. Since that's where the digital subcarriers often line, the
digital signal ends up as sideband noise in the L-R difference signal,
and (unless you switch the tuner to "mono") ends up in the left and
right channel audio.

There are a number of ways to fix this. One is to filter the FM
detector output signal before it goes to the MPX chip... either a
low-pass filter (knee somewhere about 60 kHz), or notch filters
centered on the odd harmonics of 38 kHz, or both.

Another approach is to demodulate the MPX signal using a design which
is inherently not sensitive to the 38 kHz harmonic regions... e.g. a
true multiplying mixer using a 38 kHz sinewave local oscillator
(rather than a square wave or switching design), or a Walsh-function
decoder using a stepped wave which has no 3rd or 5th harmonic
content (some Sansui tuners do this).

Modern DSP-based MPX decoders could do any of these.

http://ham-radio.com/k6sti/hdrsn.htm has a nice overview of the
problem and solutions.