4 or 8 ohm connection for a 6 ohm speaker?

I have a pair of Thiel CS2’s that have a nominal impedance of 6 ohms. My mono blocks have output impedance connections for 4, 8 or 16 ohm speakers. Is it best to connect the speakers to the 4 ohm or 8 ohm connections? 
Thanks for your thoughts.

Bill from Nova Scotia