I like to reinforced KEW comment above, with addition of a link and another option for placement of the subs. However, I would follow the process explained in the link, positioning one subwoofer at a time in the favorable locations.
Here
If possible, you can try both methods, and from there you can move on to the other step, changing out your subs, or adding subs for more out put and balancing the low frequency response across your room.