When you say 'Not as good as it should be' was it ever good, what has changed, and do you know whether the equipment is working to specification?
Whilst possible, I would expect the electronics not to be the reason, as modern electronics (I call anything made in the past 40 years modern!) generally either works or it doesn't. Faults tend to be obvious in nasty distortion, or silence, not just a feeling it's not as good as it should be.
Loudspeakers are another matter, as there are several mechanisms that could be acting:-
1) If the tweeters use ferrofluid, that could have gone hard after 30 years, reducing treble output.
2) If the crossover capacitors have changed value after 30 years, the crossover frequencies will be wrong, which will affect the frequency response. Electrolytics tend to go up in value with age, with an increase in ESR.
3) It's possible the crossover resistors have changed value, especially if the loudspeakers have been overdriven, with again a change in frequency response.
4) Is the surround on the bass drivers still intact, or is there some damage?
It's pretty near impossible to diagnose something like this remotely, but I would start with the loudspeakers, see if you can use another amplifier, and use the headphone output of the MacBook (assuming it still has one) instead of the DAC (or use another source altogether so you're only changing one thing at a time) then work back to the source.
S.
