You might want to see my post in the Transformer Size thread. I'm willing to bet the 770 cannot manage 300w x 7 continuous power with all 7 channels driven at once. This would require over 3000 watts of AC line draw. Of course, there aren't any other 7 channels amps I know of that can manage that feat either.

If you can run a 220 volt circuit, a big Cinepro amp will easily outpower the 770 but it's also about 3 times the price. Likewise, the Sunfire Cinema amps are more efficient so they can manage more power on a typical 120 volt circuit but they don't make a 7 channel model and some question their sound and reliability.

If all your speakers are 4 ohm models (or inefficient/difficult to drive), and you like it loud, I'd seriously consider using TWO power amps running on two dedicated AC circuits. You can only get so much power out of one 15 amp power cord.

As for the power doubling when you go from 8 ohms to 4 ohms that's usually somewhat a game of specsmanship. A perfect amplifier would indeed behave that way, but even the amps that are rated to do it, don't really test that way. An amp rated for 200w/ch at 8 ohms and 400w/ch at 4 ohms (like the Sunfire Cinema Grand) will in reality usually put out like 250 - 300w/ch into 8 ohms and say 405w/ch in 4 ohms. So while it meets the spec, it only does so because they (oddly) adjust the 8 ohm rating downward to make it appear like a "perfect" amp.

As applejelly posted, the 770 is respectable with its 50% increase into 4 ohms. The catch is, I doubt it can do it with all channels driven. But that's not really the amp's fault as much as what you get from a single 120 volt wall outlet and a shared power supply.