Here's another .02
As well as I can explain what I was told (I think by Scott or Steve) the 7700, sharing this enormous power supply, allows channels which require a lot of power during loud scenes to "pull" or use from the "pool", as others (maybe surrounds) are not demanding.

I'm not sure I'm explaining it right.
The other reasons above are correct. I was initially interested but when I learned about the stacking, the extra room it takes, and then thinking about the power cords.....well....I'm happy with my 7700 in one room and 755 in other.
HTH
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Living Room:
5.1 Surround and 4channel inline room
990/7700/6-KEF-107s/LFM1 x 2/ SMS Awaiting Trinnov
Millenium dts decoder;Digital Director
Players: Tascam CD01U/SonyCX455 x 3/DV955/BDP83
Old Sony 60" SXRD TV
Zone 2 (also liv-Room: listening to music while Mrs watches TV): Crown SL2 preamp/D40 Amp/Stax Headphones



My "Man-cave":
4 channel-only inline room. No TV (thank heaven)!!!
990/755/4-KEF 107s
Tascam CD01U/dts decoder/digital director
Alesis 16x4x2 mixer
Recorders Alesis HD24/ML9600/Crown CX844s/SonyDAT/Tascam DA38
Ham Radio Shack (KB1STH) ICOM/Yaesu/Drakes x 3