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

OP   "my system seem " bright"

Find how much bright your system sounds.

While your audio system playing your favorite music, say "hello" in low voice and listen your voice and music.  Alex/WTA

Could be the speakers themselves.

Since you ruled the amp and preamp. Maybe the tweeters are somehow damaged. You would not expect Revels of that level to be the problem but again it’s possible.

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.