Audyssey does not set anything pertaining to speaker resistance and does not measure that. If you had that option all it would do is cut power to the amp section to prevent overheating of the AVR.
Some AVR's are rated for 4ohm speakers but the A300 does not mention whether the A300 can drive 4ohm speakers. I use a Denon 1713 (very equivalent to your A300) in my bedroom stereo setup with a pair of 4ohm rated bookshelf speakers and a sub - it does just fine. I don't advocate 4ohm speakers with your AVR - or mine. I play the music very low and the Denon is quite happy.
As to the total power consumption take a look at the owners manual, this is a quote from that manual (pg 128):
Power supply: AC 120 V, 60 Hz
Power consumption: 360
Power consumption in standby mode: 0.1 W
Power consumption in CEC standby
mode: 0.5 W
Power consumption in network
standby mode: 2.7 W
Denon does what every other AVR manufacturer does - they take the 175w (one channel driven into clipping!) and times it by the number of amp channels (five in your case) and VIOLA! they have a real powerhouse 875w AVR. 175x5=875. Deceitful? Maybe - but 'the other guys are doing it too' is what they'll tell you when you understand what they are doing.
Amplifier power and SPL (sound pressure levels) are a logarithmic function. Your speakers might play at 90db
at 1 watt at one meter distance (2.83v). To get 93db from them takes a doubling of power - same for distance - move back one meter and you'll need to double the power for the same SPL at the new distance. Doesn't take much power to get to 102db (very loud). So at 93db we have 2w - at 96db you need 4w - at 99db 8w - 102w takes 16w. Doubling the power nets you another 3db. If you listen at reference levels (85db
) you shouldn't have any problems. If you want to play it louder you might experience thermal shutdown.