I love and own the ProAc D48R. I am simply stunned every time I listen to them. I have never heard the 3.8, so I cannot compare the two. But if you are looking for a dynamic speaker with vocals to die for, then these should be auditioned. I have not hear the K6, but guess that since they are in the higher end range compared to my D48R, they should be even better.
Thinking of upgrading speakers from ProAc 3.8 I have enjoyed in my system for 23 years.
I am likely to buy slightly used. I have read the newer ProAcs have tighter base and are easier to place in your room due to down facing port. Are the ribbon tweeters that much better? I am concerned about durability of the ribbons, as there have been a few threads by people who have had ribbon failure. I would consider D48R or K6 if I can find a nice used pair. Do the newer ProAcs lose any of the magic of the older models? Another speaker that interests me is the Sonus Faber SarafinoG2.
System is Accuphase E406, Nottingham Spacedeck, Allnic H5500