What I remember from school (long ago!) is that the cosine factor comes into play for these power ratings.
Also, if you run this amp at full power consider the efficiency- it's surely never over 45% for class B amplification.
Nice discussion though...
But never fear, I am building my own 7*240W @ 8 Ohms final right now based on Coldamp www.coldamp.com modules, and they said power ratings could be relaxed greatly for the surround channels, as the bass channels (my fronts are biamped) are the most power hungry.
Our power grid in Holland is 230V, but that should not make any difference for this discussion, only the amperage is less of course.
BTW- My 7700 is expected on Thursday, and I plan to rewire the transformers' primaries for series operation (effectively 230V), keeping my fingers crossed...
Or should I worry about the load per transformer obviously being different as they feed different amps?
And there's the 50Hz vs 60Hz issue, now what I remember running a 60Hz designed transformer on 50Hz results in the transformer getting warmer or was it the other way around?
Suppose the 990 has only one, so the trick won't work here, but I have a hefty step transformer ready to be used.