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Before I bought S-400, I had tried ProAc D28/D18 also. I finally picked S-400 as it gave me more all-around music with my current setting (in my 10 x 16 ft room).
My 150X (tested in Radar with 155XS) is about to manage S-400 smoothly, of coz, NAP200/250 will be much better I believe. Matching with D28, 150X can't manage it especially the high is "bill" and low is fat. With D18, it gave me more bass than S-400 (can't believe it), however, it was more hifi sound. If you like high res and more details hifi sound, D28/D18 will deliver better S-400.
Compared to ProAc, S-400 gives you sweeter vocal, tighter bass (you can say less bass), but not fat. It gives you a consistent/smooth vocal and mid to treble range (I believe it is because how BMR performs). S-400 won't give you a super detail/sound stage as others but it gives you much better overall musical expression for you. I am not saying it is not a good speakers. It just lets you listen longer but not tired. Mine runs more than 300 hours now, it's already given a very open sound stage.
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