Here is another comparison, this time with left sub, 2 subs and no subs and a bit more smoothing (1/12) to make this more clear.
Great pic. Um, you definitely have complications. There's an increasing trend from 60 Hz to 1 kHz. This should be flat or descending. Also, notice the regular ripple starting at around 6 kHz. That indicates regular reflections in the mid-treble.
Essentially you have way too much mid bass to mid energy. If this is a highly reflective room that would explain it. Before adding the sub, fix up your mains and room.