Looking at the freq response. There are two things you MUST do to get dramatic improvements to your overall listening experience:
Go dual subwoofer, Period. No other way out
You are losing SPL as early as 74Hz. The critical range for that mid-bass kick is from 70Hz - 100Hz. You are missing tons of SPL. To reclaim those “missing” SPL, pls get another subwoofer, preferably the same make, model and driver size. If you can’t go for the same 12" enclosure, then go for at least 10" of the same make and model. Place the 2nd subwoofer diagonally across the room (preferred) but if you can’t due to space constrains, put it at the same 1/4 pt from the left wall, making flushed with the LCR.
You have a room mode issue
Either move your speakers or your main listening position (MLP). The constant dip from 80hz onwards is showing up for the entire speaker arrays and not just the subwoofer which points to room mode. No amount of EQ will fix that. Use the Harmon Room mode calculator (see my post: Getting Good Bass for your Home Theater Setup - #6 by desray) to determine where to shift your MLP. Bottom line is NOT to place your listening position at the null location. It will improve your listening experience drastically and even fix your subwoofer and speaker freq response as well. Guess what, this ,method if used correctly, you don’t even need to spend a single cent, except for the 2nd subwoofer that I firmly believe is a necessity for you.
Hope it helps…