I made the same move from the arrivas to s400 a year or so ago with absolutely no regrets at all!
I use them with a 202/200 and it's great! it was a really good upgrade and I can't see I will ever change them. it's a totally different league as you would expect from the price tags.The s400 took a while to settle in but I couldn't be happier. I managed to get £350 for the old speakers on a private sale which helped.
The only hitch is whether or not the 150x is powerful enough or not, and you need to test this first!
The s400's take 2 people to unwrap and set up so home demo which is so essential with speakers isn't that easy. They are really a lot bigger than the arrivas and borrowing them to try quite tricky i should think. I use mine in a 5x4 m room and they do dominate the room visually - my wife is quite patient….
Good luck