I own both a Ref 5se and Ref CD7. This is a wonderful sounding combination. I am sure the Ref 6 and CD9 combination is even better. There is a synergy in an all ARC system.
A friend had a K-01 and it was quite nice. He also owns a Ref 6. He just sold the K-01 and I am not sure what he is considering replacing it with. I have never heard a system that didn't benefit from a really good active preamp and the Ref 6 is world class.