My own experience with 6k's is that you will never need an LFE. And that the 55k is the easiest path to a center than can play with the 6k's. I think others have gone the SE route. YMMV.

main room: 861v4, 2x6000L/R (24/96) 1x5500HC (24/96), 2x DSP33(Rears)
MS200/Roon, 598, Revox b710mkII tape deck, Revox b760 FM tuner, Revox b790 turntable
bedroom: 568.2mm, 3x5000(LCR) (24/96)