are your mains angled inward or straight out? I found by adjusting my 80's further away from the side and front wall, and playing with toe-in, it helped that dip, as did the bass traps.

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85