The best speaker for such rooms to compete with some of the best sounding rooms is the Western Electric 16A replica with GIP drivers. It can be on the long wall, and it sounds same 2 feet away, 10 feet away, and off-axis. It was made to address an audience of 800 in cinemas and to also sound great for those sitting in the first row corners.
I recommended this to an audiophile buddy in London and he just bought it into his 4m * 3m room. It has no room problems right now, he is using cheaper drivers though so you won't get the GIP magic
It has the best tone and magic I have heard till 75hz. Below that, it needs to be crossed over to a sub.
115db efficiency