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.