they sound similar, the x350 being more dynamic.
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All the pass amps are wonderful and they are now going to have a version of the X 150 with the same faceplate as the X 250 and X 350.
Theoritically the X 350 having a little different design should be superior but they sound so much alike you will never tell the difference. If you listen to your music at low volumes you will get more low level detail because of the extra power. Otherwise it should make no difference.