Hi Kushanava,
I used my x-12s with both an M3i and and A3 pre and power setup I have.
I did not find them bright. Mine were primarily used in a damped room. My room is an extra bedroom that is more of a library, with wall to wall carpet and bookshelves all around. I sat between 6-9 feet away and pointed them straight at me with the speakers about 6-7 feet apart. I didn't really listen loudly though. Maybe 9-9:30 on the volume knob, if that gives you a reference point.
A couple of questions: are the x-12s you bought new? I bought mine used, so mine were broken in already, if that is needed with these. If yours are new, maybe they simply need to play for a while to loosen up.
What is your listening space like?
Also, kind of obvious, but worth doublechecking: is the polarity correct on the speaker wiring?
Do you leave your amp on or turn it off after listening? I left mine on all the time because I found it took 30 minutes or so to sound best.
What is your source?
Other than that, I'm not sure what else the problem could be. I thought the combination was crisp and clean with firm bass, but not bright. I may also be more tolerant of treble energy than you are.
It may be simply that you want something warmer, like Spendor or Harbeth speakers, or maybe a warmer amp. When I used my x-12s with a little Naim Nait 5i I have, it sounded more full in the midrange and the treble was less pronounced.
Can you return the speakers?