Next is to buy some decent bass traps for the corners. Any recommendations?
Whatever you buy has to take out two significant problems (assuming that the dimensions you gave were in feet), and that willl be the pile-up of modes at two frequencies, one of which is 56Hz and the other of which is 113Hz. If you can knock these two down, you will have solved the worst of your problem.
If you use AA to create a slow sweep through from about 40Hz to about 150Hz, you will probably be able to hear the effect of these quite clearly.
Oh, and FWIW, if the walls are bare and plastered, you might need a
little more than 100 sq ft of foam - but this is heading in the right general direction.