Nick, the most significant difference in avoiding/cancelling room modes for smoother overall bass response occurs with two correctly-positioned subs. Basically, from research by Dr. Toole and others, this is at the mid-points of the side walls or front and back walls, or diagonally opposite corners. Four subs do make for some further improvement, but less than going from one to two.
The most cost-effective change for you would appear to be to add a second sub similar to your LFM-1. More costly would be to instead get two subs which had more bass extension than the LFM-1.
