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. 

Again, I would recommend having your ears checked.  What you describe might be the start of hyperacusis, an ear condition.  

If it is an equipment issue, my bet is on the speakers.  But, it would be extremely unlikely that both speakers have the same degradation, and to the same degree, so listen to each one alone to determine if there are differences between the two.  If they sound the same, the problem is probably elsewhere.

I am glad it is not a hearing issue.  Now, to diagnose the problem.  Put on something that is a mono recording which exhibits the problem.  Use a balance control, or disconnect one speaker to listem to one channel at a time.  Is the problem the same for both channels?  If there is a difference, try switching the left and right cables to the speakers.  If the problem stays the same, it is the speakers that are at fault.  If it switches channels, it is something upstream of the speakers.  You can move progressivel upstream doing this kind of switching to isolate which component is at fault.  If you have problems in both channels that is about the same, it would be an odd sort of failure, particularly if it is the speakers at fault—it would mean identical damage that is short of taking out the tweeters entirely.

The fact that you’ve had multiple tweeter failures suggest that you are over driving your speakers or that something like your amp is defective and maybe sending a lot of ultrasonic energy to the tweeter and damaging it.