Actually, you bring up the prime reason I found that 7.1 can be an improvement over 5.1, but that it doesn't necessarily work for all rooms.

If your couch *is* up against the back, then 5.1 is probably the best you're going to get, and any attempt to do 7.1 might actually degrade the sound field. (Where are the last 2 speakers going to go?)

But if you *do* have space behind where you sit, then I believe that 7.1 can be a real benefit. The reason why, is that those last 2 speakers in the back fill in a gap in the soundfield you don't even know exists, until it's been removed.

I had a friend over once, and he wasn't convinced of the benefit of 7.1 either. So we picked some movie sequences, and switched back and forth between straight DD in 5.1 and 5.1 DD with the CES mode engaged (7.1), and even he admitted that the benefit was there.

One other fact is that people do not hear as well behind them. So in effect, more speakers are necessary to help out our natural limitations.

And, think of real life. Sounds come at us from all 360 degrees around us. So the absolute best home theater would actually have an infinite number of speakers, all around us.