I certainly am no expert reviewer but I've owned the 400R - it is dynamic, transparent and has enormous down low bass that is textured and tight. Very, very nice power amp indeed. I have always heard from other's that the Synergy amps have a smooth, tube like sonic signature. I listened to a 450 a long time ago and I remember thinking it was fantastic. There are others here on AC who are more deft at explaining the differences but I don't think there is a wrong choice here.