Thanks av_phile1 the seller also recommended the 8 ohm setting but couldn't give me a reason just that for him it sounded better....
Power amps (both tube and ss) can be seriously damaged when using the wrong impedance loads.
For Tube amps, what causes the damage is
underload. That's why tube amps should not be operated when there is no speakers connected. So if you happen to have a 6 ohms speaker and your tube amp has 4 and 8 ohms output taps then you better use the 8 ohm tap instead of the 4 ohms tap. Connecting a 6 ohm speaker to the 4 ohms output tap of your tube amp presents a slight underload condition to the secondary of the output transformer but should'nt be enough to destroy your output transformer.
Otoh, Solid state power amps can be easily damaged by
overload conditions. That's why unlike tube amps, they can be operated for extended periods without any speaker load. Connecting a 4 ohm load to an ss amp designed for 8 ohms will not destroy it at once but will cause the output devices to conduct more current hence more heat. If you don't take care of this too much heat, then your ss amp will go to thermal runaway and destroy your output transistors. So to sum it up, an ss amp designed to run 4 ohms loads can safely handle 4, 6 or 8 ohms, but if the amp is designed for 8 ohms loads, it would be safer to at least use 6 ohms loads and never 4 ohms.
Now for 6 ohms speakers, I don't think it should be too much of a problem connecting them to any power amp, be it tube or ss.
Mixing impedances doesn't matter, such problems usually arise with HT setups. What I mean is your fronts can be 8 ohms, your center can be 6 ohms etc. What matters most is that all impedances should be within the output impedance specifications of the receiver.
Cheers