Chris Hornbeck:
My first guess would be bandlimiting, protection from RFI, etc.
Out-of-band signals cause in-band degradation in lots of places,
especially phono equalizers, DAC's and mic preamps.

JB: Boy, there are a lot better ways to do those things w/out degrading
the sound.

Scott Dorsey:
I'm not sure there are. I really don't like the coloration transformers
add, but I'm often willing to put up with it because there is just no other
way to get that kind of CMRR. You can deal with outrageous ground
differences between gear using transformers and in field situations that
is sadly a requirement.

I'm talking about sixty or seventy volt ground faults... not the kind of
thing that typical active inputs deal with very well.

But Chris didn't mention CMRR or ground-potential differences. He
mentioned bandlimiting, which can be done with a shunt cap; and
protection from RFI, which can be done with good shielding practice &
ferrite beads (possibly not ideally, but still quite powerfully).

If you have 60- or 70-volt differences in ground potential, don't you
have a defective power transformer somewhere?

James Boyk