To echo what others have said the newer Proac models with ribbon tweeter retain all of the qualities you like in your 3.8 models and sound better with improved imaging and transparency.
I have the Proac D-40r, and have owned them since I purchased from the local dealer as demo speakers. I traded in the Proac 2.5 which sounded great, but were less efficient and more veiled (which I didn't realize until I tried the new speakers.
There have been no issues with any of the drivers, ribbon or conventional.
During my time in this hoppy/obsession I have owned a lot of different speakers, but I have owned more Proac speakers than any other brand.
I haven't heard the D-48r or the K-6 so I can't help you with that question.
My system is posted if you want to look.
Jim Perry