The peak at 38Hz area is likely causing a lot of energy in your setup levels for REW and may result in the overall level of the plot being somewhat low in relation to the target.
So, if I overlay a target on your graph that has been dialed down a bit from 75dB, the graph to me looks like a large room peak at 38Hz and a dip at 50Hz. Fairly normal.
I would work on the peak first and then do some re-measuring. I think you'll find the response will be better.
You could also drag the sub to the middle of the room and do a near-field measure and see how it looks...
test1.jpg
brucek