4ohms or 8ohms for refurbished WATT/Puppy 5.1's?

 Hello,

I'm hoping for some guidance here.  I have a McIntosh MC2100, McIntosh C28, and WATT/Puppy 5.1's (7 tweeter and 6 midrange, they are refurbished).

My question is, on my MC2100 there are posts for 4ohms, 8ohms, and 16ohms, and I'm not sure which posts I should be using.  The seller told me they are 6ohm speakers, but that still doesn't do much in terms of helping me understand which one to use.  

I have done some research and listened at both 4ohms and 8ohms, and it seems like 4ohms may sound a tad better. 

I am not very well versed in this realm, so any insight would be very much appreciated!