Might be a bit small for my room, 10 x 6.5 meters.
I see your room is similar in dimension to my own, also with S-400 but with an Nait XS driving them. Do you have a primary listening position this room, how are the speakers oriented?
As background I tend to treat my hi-fi as a glorified radio in that I don't get a lot of time to still to sit in particular place and just listen to music. I'm always moving around/through this room and in this context the S-400 are exemplary as the music sounds good wherever you are. That's important to me, as I consider myself as someone who is more interested in music than hi-fi. However there is still an area where the music is appreciably more focused. In my room I have the speakers 3 metres apart firing down the length of it and the sweet spot extends from 5 metres out and continues to increase in focus as you get closer to the 2 metre mark where the focus becomes clearest and dare I say it punchier. Though I am often tempted to upgrade the amp, I suspect I'd be betting a much bigger upgrade by giving up work and turning my much smaller study into a listening room and spending more on source.
good luck with your choices.