Maybe there's something in the older Amigas running slower, and the source not coping
well at the high end, I'm a bit out of bounds there without much knowledge of Paula.
If the hardware filter is just a resistor capacitor network,
that could be found out by removing it and using a modern hifi.
Don't know how it translates to UAE if it's still a Paula emulation trying to do the same
thing, at the same slow clock speed.
I do have a 1962 stereophonic valve amp that has been modernised with digital optical input.
Typically, the audio valves in the amp compress the audio somewhat,
and the audio output transformers typically aren't even capable of delivering the
very high or very low end by today's standards.
So you can have come pristine audio from a class A amp on the cheap,
but have to be selective about the source music, because it can still be noticed
if it sharply cuts some high frequencies... which varies track to track,
and it's now a "shelf unit" with midrange drivers only,
but some analogue processing to push the spectrum into those driver's range.