sam
my honest answer is i don’t really know, as i have not heard the k series, and also i don’t know how you like the tonality (specifically bass heaviness/impact) of your system - some folks really like bass!!
your room is plenty big, so either k6 or k8 shd work fine and not overload, also depends what you drive them with tubes or solid state... i would say, and can say though, that my d30r has, by my ears, a very solid low end in my 16x18 room, and i think stuart is clearly programming in a very solid bass foundation in his modern floorstanders... much more so than his earlier ones like response 3 or 3.5 which were clearly bass light despite the dual woofer design (the notable shift occurred with the r3.8 - responding to complaints about weak bass, he traded a few pts of efficiency and the friendly 8 ohm impedance for more solid fuller bass response)
ps - mijostyn’s point is a valid one... sats and subs can be a very very good sounding solution for proacs... but just depends if you want to go floor standers for sonic, aesthetic or simplicity reasons vs high performance sub/sats
