Diffusors vs. absorbers room treatment

Hello experts,
I'm trying to toy with room treatments to see how it will improve my system. I just moved my system upstairs to a smaller room and I notice the sound is a little brighter than I would like. Can anyone tell me a basic or rule of thumb on where to use the diffusor and where to use te absorber? Or are they pretty much the same thing? I visited few website from like sonex, auralex, rpg etc. I am thinking about getting sonex classic since I thought they're pretty reasonably priced. Can anyone also recommend me some other that are not toooo expensive? My room is 14' wide by 13' deep. I place my sonus grand piano speakers about 2' from walls. Thanks in advance.