Chooser, the basic 7 channel, 120 watt, 20-20KHz, 0.04% THD, 8 ohms rating of your 2400 can be relied upon as accurate since it's governed by the FTC power regulations and a manufacturer would be foolish to violate the regs with bad numbers. Under the regs the rating for a multi-channel unit has to be taken with at least two channels driven at the full rated power for at least five consecutive minutes. If nothing is specifically said, the rating is to be understood as being with two channels driven; manufacturers have the option to test with more than two channels driven(e.g. "all", although this isn't a realistic scenario for home, rather than laboratory, use)and will state that as part of the spec. However, the 500 watt power consumption figure has no connection with the maximum power rating. Unless the power consumption rating is specifically described as e.g. "maximum", it's an average figure derived during a UL required test for overheating with all channels driven at 1/8th power(considered to be roughly the average power consumed over the course of playing an entire CD). Although class AB amplifiers are roughly 50% efficient at full power, efficiency falls steeply at lower power levels and is roughly 20% at 1/8th power. 1/8th of 840 watts(7x120)is 105 watts, and at about 20% efficiency will require about 500 watts of consumption. So, there's no inconsistency between the seven 120 watt channels and the 500 watt consumption figure, although we read comments to the contrary by those uninformed on the matter
A little on power peaks. The IHF Dynamic Power rating is derived by using 20 millisecond tone bursts repeated every 500 milliseconds. Although this varies somewhat with the instrument involved, 20 milliseconds was adopted by the IHF as being a typical number.
Note again that if you'd like to know the requirements for power, that the two sources from Voelker and Linkwitz which were given earlier are very useful, since the entire sound pressure level from combined direct and reverberent sound has to be taken into account to get a realistic number. Since over half of the sound reaching a listener in a typical seating position comes from reverberent, rather than direct, sound, considering only the 6dB decrease of the direct sound for each doubling of distance doesn't describe the total situation.
