My music sounds brighter

@cspiegs,
Interested in update of any progress or elimination of causes to your problem.

@erik_squires,
Spot on with analysis...


@dweller,
Spot on with your analysis also, I have found alien a/c can reach into places past where you can adjust from menu. Try unplugging gear for minute or two and check again...

@giri,I hope you are mistaken and it is not that serious...

Elizabeth

I am all for the ear wax theory. It could also be ear hairs. 

Continually there are changes in our environment - everything from changes in temperature, changes in humidity and air pressure to fluctuations in microwave transmissions - that all have a slight affect on sound and an additional effect on how well we hear it. All of these same things, along with allergy flair-ups, inflammation, blood pressure fluctuations and even minor infections, can have an affect on inner ear and sinus pressure and how we hear at any particular time.
There are some nights I wake up and hear the tic-tock, ticking of both wall clocks in the rooms adjoining my bedroom - almost to the point of wanting to cover my head. Other nights I can barley hear them and sometimes, no mater how hard I listen, not hear them at all.
It could be an unaccounted for change somewhere in the chain of your audio system, but, just as likely, some type of change in your environment or in your self....Jim

How old are the speakers ? If a inner crossover part is going 
Such as a capacitor  that will deduct something from the musical performance. That includes electronics, And does it happen on everything such as cd,records
Even  s new wire or line from your circuit breaker will change 
Sonics. 

I suspect it's the amp. It may need servicing since it is 20 years old.