I would like to chime in as a manufacturer of Modified Schroeder Acoustical Diffusor panels.
It is a myth that diffusors do not work in small rooms. I have put our professionally designed diffusors in many small rooms (are there any large rooms left?). The proof is in the pudding as they say. The clients have been overwhelmingly positive, their jaws usually drop as they hear the change in the acoustical environment the diffusors create as they go up one by one. They can hear the not so subtle change in the sound of their own voice. Their room sounds large and there is air around the sound. Acoustics is a theoretical science and the final proof comes upon completion of an installation. You have to use acoustics theory as a starting point but you also have to recognize the effect of applied science.Check out some examples at
http://www.srlaudio.com/acoustics.htm
I agree you have to first deal with the bass using corner absorbers. It is also a good iidea to have a "sprinkling" of absorption panels in key places. After a studio is up and functioning with the diffusion modules, I make it a point to ask visiting engineers what they think about the acoustics. The usual reply is they had not noticed them, this is the greatest compliment because they are not hearing the room, it is not a distraction, they are not compensating.