The sound on my A1200 desktop isn't too crisp either.
If I turn the volume up to max on my external speakers I can often
hear the CPU when it's working on something.
Like an audible CPU monitor...
;-)
Do you think a ferrite core on the RCA leads would help or is it more
to do with capacitor age/quality?
I've also heard that the serial port is somehow linked to the audio
out phono jacks for MIDI reasons, is there a -12v rail that is
susceptible to fault causing audio trouble?