Another thing to do is check the power supply value for both your current amp and the new one you are looking at as this will give a more accurate figure for the total power output they will give. ....
This is the point I was going to make. The REAL power can never exceed the Power Supply rating. It is not uncommon for a AV Receiver to claim 100w/ch for a total of 700w, yet it will only have a 400w or 500w power supply. Yes, when you driver two channels, you can get much higher power, maybe even 200w/ch, but when you try to drive 7 channels heavily, the power drops to 72w/ch. That is just a fact of life.
Now, they aren't really cheating because the Surround Channels are not driven any where near as heavily as the Front and Center Channels. They carry the real weight, and take the real power load.
According to the owner's manual of the Onkyo TX0NR818, the power is -
(North American)
135 watts minimum continuous power per
channel, 8 ohm loads, 2 channels driven
from 20 Hz to 20 kHz, with a maximum
total harmonic distortion of 0.08% (FTC)
FTC is a solid and real unambiguous power rating to a standard speaker impedance load full spectrum with minute factional Total Harmonic Distortion.
So, 135w x 7 channels is 945w total. However, to the best of my ability to determine, the power supply is 730w. Meaning with all channels driven, the single channel power can never exceed 104w/ch. Though that information is not clear and direct, it took a degree of interpretation.
In other parts of the world, for what ever reason, they fudge the power rating by making them -
(Others)
7 ch × 180 W at 6 ohms, 1 kHz, 1 ch driven
of 1% (IEC)
Notice NOT to a standard Load, NOT full spectrum, and NOT to minute fractional THD, and we really don't know how many channels they are driving. It could be just one channel. In short, that is an inflated power rating.
As to your original amp, the Onkyo TX SA 805, unless you still have the owner's manual, and unless it gives clear specification, we can't really know what the true power was of this amp.
But do keep in mind, a GOOD 50w/ch stereo will likely drive speakers better than a 100w/ch AV amp. Plus, once you get up near 100
real watts, the quality of the amp means more than the power. In a more expensive amp, you are not buying power, but clarity and drive capability. A strong high current 100w is a lot better than a weak modest current capability 100w amp.
Quality buys you clarity, not power.
All AV Receivers have enough power, so concentrate on the quality of the amp, and the power supply. Many Consumer grade AV amps are not 4 ohm rated. That is, they need speakers of 6 ohms or 8 ohms. If the front three are 4 ohms, because of higher current draw, they are going to have a hard time keeping up.
Also, because they are cramming so many amps into such a small space, cooling can also be an issue; more power consumed means more heat. But there is sometimes a limit to how fast that heat can be dissipated. If you are driving all channels hard, to low impedance loads, your amp might overheat and shut down. In this case, it is a combination of cooling ability and the power supplies ability to supply high current demands.
Also keep in mind that some amp maker rate their power will ALL Channels driven, meaning that those lower power ratings reflect Real Power delivered under any and all circumstances.
If you are serious about REAL power, then plan on spending some REAL money on a bank of discrete Stereo Power amps and a AV Processor/Pre-Amp.
Short of that, focus on amps that are noted for having high current, high powered power supply, and generally high quality amp components.
Just a few thoughts on the subject.
Steve/bluewizard