What sort of sub do you have? A pair of 755's could drive seven channels plus a pair of big passive subs (the SV subs are typically a 4 ohm load, I believe -- and 300W @ 4ohm is pretty stout) and still have an extra channel. If you have a powered sub (or don't want to wait until you add surround back speakers to have any amplification for a passive sub), I'd probably lean toward the 770 now rather than the 755 -- it's not much extra cost now (especially with the $200 package discount!), and you'll be ready for the extra speakers. It is sort of a tough question, though...

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Gonk