I'm one of those guys with a pretty decent system and a problem room. It's big enough for big line source speakers, but it has an open end, a split alcove, tile floors, 14' glass window on half a wall, and the list goes on...
First thing in my favor, BY FAR, is a very
tolerant wife ( the most important component in my system). So large bass traps are in place, huge absorption panels are behind the GR LS9 speakers and on the alcove. Pi Audio diffusers are in place between the speakers, in front of the bass traps and to the side and slightly in front of the speakers. Much of the placements were on the advice of the very knowledgeable D. Elledge. Most of the diffusers can be put up quickly, or taken down to watch a movie with the tolerant wife.
Is it now a great room? No, but now it approaches the sound that Danny Richie heard when he designed them (the LS9 line sources). I'll never spend the bucks to have a "dedicated" listening room. But with a little help from my friends (or a lot of help), I've been lucky enough to put together something good.