I just looked at the owner's manual, and it says it consumes 420 watts of AC power at 1/8 power into 8 ohms.

The rated maximum output at 8 ohms is 300W per channel, so 1/8 power would be 37.5 watts output per channel and 75 watts total.

This indicates that in that condition the amplifier is operating at an efficiency of 75 W/ 420W or 17.8%. 75 watts is being delivered to the 8 ohm speakers, and the other 345 watts is being dissipated as heat by the amplifier.

This kind of operation would be typical of that type of Class AB amplifier when played fairly loudly into typical speakers. PEAK power may be much larger instantaneously, but average power will be much lower; perhaps 20 or 30 watts typically.

I don't see the 736 watts figure anywhere. I suspect that may apply to a larger amplifier, not the CA-2300.

It might also be useful to point out that when a Class AB amplifier operates at higher power levels, its efficiency typically goes up to around 50%.

This would imply that IF the CA-2300 was operating at 300 watts per channel with an 8 ohm load, it would be putting out 600 watts to both channels, and would probably draw around 1200 watts total from its power source. This would only occur momentarily in normal use, not continuously.