You won't go wrong with either speakers. From some of the reviews I read the D-48R's are more euphonic whereas the K series is more detailed and neutral. I think it does come down to personal preference. Personally a big fan of the D-48R when I heard it but had to "settle" for the D-30R. due to room size. No regrets however. I think both speakers offer a lot for the money.
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