Yeah, square rooms are horrible. For higher frequencies you can try a skewed speaker/listener layout, but bass is the real problem in most rooms and especially the square ones. Our (AC) normal acoustics vendors, Glenn and Ethan, would be your best, low cost source for advice.
Yeah and dump any foam (unless you like the look

), it's pretty much useless, again especially for lower frequencies.
And BTW, diffusion only works for higher frequencies too (1,000 Hz and above).