The 7700 and 7500, like many multichannel amps, are somewhat limited by the capacity of the electrical circuit they are fed from. A 15A circuit on a 120V system can in theory provide 1800W of power to a connected device, but in reality a 15A breaker is designed to trip before it reaches that level - probably before 1500W (12.5A). That's why the 7900 (300Wx7) has two separate power cords: you are supposed to connect each cord to a separate dedicated circuit to provide enough current to be able to achieve the amp's full potential.

You also have to take into consideration efficiency, as no machine (mechanical or electrical) can be 100% efficient. (Hmmm... "perpetual motion amp"?) That means that even with 1500W of input to the amp, you are only going to get something less out. The rest will turn into heat. How much less is difficult to say. The reason that this works out OK in practice is the nature of the source material that we are listening to. The only way to drive all seven channels to significant levels at the same time is to feed each channel a very loud test tone at the same time. Normal situations are quite different than that, with different channels having different needs. You could have the center channel (as an example) driven to "reference level" - and if you had very inefficient speakers in a very large room, the speaker might demand 190W from that channel. At the same time, though, other channels would not be seeing the same sort of demand, and you might only be delivering eight or ten watts to the surrounds.