** Man of Leisure **
The peak could easily be caused by a room mode. Placing treatments at the first reflection point will address reflections coming from that spot, but reflections and peaks in frequency response are not the same thing. Peaks like the one you have are usually addressed by speaker placement, or by EQ. Are you using any type of EQ, like Audyssey or perhaps a Mini-DSP? If not, have you considered EQ?