bug: you pose an interesting question.
If you wanted to test 7 channels, all at the same time, at a single tone to 300 watts each, you certainly would need more than a 15 amp circuit. No doubt about it!!!!
But, is this a realistic test for the HT environment?
My answer is NO!!!!!!!!!!!!!!!!
If you look at the decibel sound pressure level example link that Ajax posted, for 100 dB, for 91 dB/ watt speakers, you need less than 10 watts per speaker. For sound pressure of 110 dB, you only need less than 100 watts.
It all depends on how loud you are going to listen. Even though I may get the 300 watt amps, I'm certainly not going to listen at 110 dB. Remember that I posted that 80 dB was very loud for me and I had to turn NASCAR down when it peaked at 87-88 dB? I really don't think that I would listen at 100 dB.
Very high instantaneous power peaks that are found in some music are handled by the capacitors in your power supply. That why some manufacterurs like to claim more capacitance in their power supplies than their competitors.
You probably do need to see how much equipment is tied to your HT circuit. You could turn off the circuit breaker and see what turns off with it. That might be the easiest way.
I suspect that the "true" test is to use all of your equipment (TV and loud HT) and see if the breaker trips. If it does, then, of course, you have a problem.
It you are going to wire another dedicated circuit, I'd go for a 20 amp circuit. It probably won't cost a lot more, and it will give you some additional capability.
Hope that this explanation helps.
