Help, my system seem to have slowly gotten " bright"

Can you confirm that it is the system at fault?  Do others hear the problem as well?  Do you also hear this problem when you visit audio stores?  I am wondering if you might be developing a hearing problem. 

The reason is especially if you have some older gear is the capacitors are starting. To dry out ,with that you willl start losing detail brightness comes with thst.

the vast majority of speakers come with stock lower quality capacitors and resistors .you mentioned nothing on your speakers ,having rebuilding Loudspeaker Xovers for years ,that’s  a place where most companies skimp in lower quality capacitors like Solen ,and resistors .

My 1st thoughts were it has something to do with the speakers. Did you move or adjust your speakers recently? Move any of the surrounding furniture? I recently had issues with fatigue after messing with my speaker placement. That was an eye opening experience for me. 

As far as equipment goes since you ruled out the amp/preamp I would wonder if the power supply of your source is going bad. Or perhaps some other capacitors as others have mentioned. 

On a side note..does adding more bass actually make a speaker sound less "bright" or fatiguing? 

Obviously my thoughts on what could be causing your sudden "brightness" are just guesses. Please share when/if you figure out what the cause is. 

If you didn't make any changes to the gear, placement, etc. maybe it's your hearing.

you have gotten older get your hearing checked before you do anything you might regret