Gonk is right
I have both. Was using 755 in Living room powering two Crown electrostatic fronts, a Legacy center and some old surrounds that are surprisingly good, esp for their age. Bought over 30 yrs ago.
I cannot say that I have heard a difference. I did not do an A-B comparison since the 7700 was bought to replace the 755 because of different setup in living room and I could use the 755 to replace four Crown DC300As which were powering four Crown electrosats in biamped mode.
So, I'm sorry. All I can say is the 755 was great and the 7700 is great (both with 990)but the 7700 is used in a very hybrid mode, which is why I needed the extra two channels.
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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