I have owned many of the krell FPB amps, including the aforementioned ones. I have also owned the Evo 402, 202 and 505 as a system. The original FPB600 amp was special and closest to the preverbial tube sound without the drawbacks. The higher power amps are actually more musical which is couterintuitive compared to other amps. x series amps indicated extended bandwidth designs for more linearity in the audible range with less distortion. The later the amps from krell the smaller the power supplies got, although they are still huge compared to other manufacturers. If you can get an original FPB600 amp for a steal then get it. Krell will support it if need be. FYI, cast does not add musicality, only convenience and an alternative to spending big dollars on interconnects...but where's the fun in that right??