6.1 vs 7.1: there is a psycho-acoustical effect called reversal, whereby sounds from behind you can be construed as being from in front of you (and 110 deg vs 290, 55 vs 235 deg, etc). It's rare but it happens. 2 rear speakers greatly reduces this effect (some say it eliminates it). Personal experience: I took 18 months to go from 5.1 to 6.1 to 7.1. 6.1 does do a nice job of filling in the back of the room, but with 7.1, it helps add discreteness between the back left and back right sides of the room. One last one: Lexicon and Logic 7. A lot of people feel that Logic 7 is one of the best HT DSP processing algorhythms available. David Greisinger, the driving force behind Logic 7 at Lexicon, feels so strongly that 6.1 is a bad idea that Harmon Kardon receivers and Lexicon processors cannot be configured for a 6.1 setup. 5.1 or 7.1 is all you get. For the above reasons plus a few others.