Understanding Speaker Output on Nikko NA-550

Hey there, this is pretty typical.  Lots of buyers like to run two separate pairs of speakers, but you don't have 2x the circuitry.  The jacks are just there for convenience.

The lower the impedance number, the harder to drive. 

The top line means "A or B" - each speaker can be rated no less than 4 Ohms.

The bottom line means "A _and_ B " - Each speaker can be no less than 8 Ohms.

Hey thank you so much for your quick response!

So just to understand this better, the notation here is opposite to that in boolean algebra, where A*B would mean A and B, and A+B would mean A or B?

What is the reason behind these different ratings, i.e. what is different between the two configurations (maybe in terms of the wiring or other internal features) that would cause the different impedance rating?

just to understand this better, the notation here is opposite to that in boolean algebra, where A*B would mean A and B, and A+B would mean A or B?

Forget about boolean algebra.
The top line " A.B-4Ω~16Ω/SPEAKER " means IF playing speaker A OR B only, the amplifier able to safely power 4Ω to 16Ω impedance rating speaker.
The bottom line " A+B-8Ω~16Ω/SPEAKER " means IF playing speaker A AND B together, the amplifier able to safely power 8Ω to 16Ω impedance rating speaker.

What is the reason behind these different ratings, i.e. what is different between the two configurations (maybe in terms of the wiring or other internal features) that would cause the different impedance rating?

Nikko NA-550 is designed to safely power speaker not less than 4Ω impedance. If playing two sets of speakers together (speaker A and B), both set of speakers are connected in parallel by the speaker switch and the total impedance apply to the amplifier will be less.

https://geoffthegreygeek.com/calculator-speakers-in-parallel/

Ah thanks that last paragraph makes perfect sense, thank you.

Yes, I agree that the top line " A.B-4Ω~16Ω/SPEAKER " is confusing, this is much better to understand:

https://www.springair.de/media/image/79/22/65/h53466-5-53466e.jpg

I have a NA-550. I have a 6 ohm pair of speakers on A and an 8 ohm pair on B. I run A+B all the time. I think this is technically out of spec, but is it anything I need to worry about?

Also, the 6 ohm pair is much louder than the 8 ohm pair. Different speakers, obviously. Also different gauge, type, and lengths of speaker wire. Can anyone suggest why there is such a disparity in the volume of the two pairs?