I unfortunately wouldn't have a clue about the centre... am only really a stereo person... that had to struggle my way through much room acoustic issues...
good practice is to get it in-line with the FL and FR drivers... which is always difficult... At a guess put it higher and test... it could mostly bounce from the floor - or ceiling for that matter... so that floor bounce could be the source of the brightness, alternatively turn it on it's side - still standing in the centre, just to check at the level of the FL and FR drivers... But honestly I am a bit out of my depth...