You are probably reflecting mid or lower frequencies which are more omni directional, and they splash into your bigger than usual LCD. Or maybe your new bigger LCD monitor is giving you more reflections into your table of mixer thus creating a combed filtering effect, distorting what your hear at mix position. Two things you can do. Put the LCD further back from your speakers, about 6 inches to 12 inches away. In other words, your speakers are in a forward position. 2nd, put the LCD on a higher pedestal. Reflections may be hitting your mixer or table which then reflects into your ears, thus you perceive brightness. Or, put your monitor speakers at ear height, on a higher pedestal, toed in, and placed further back from your table or mixer, making sure first reflections do not hit your table or mixer top. Do various combinations until your perceived boost on high frequencies disappear.
Tell us how you solved the problem. We would also like to know how you did it.
FWIW