Sorting Out Versions Of Krell Amps

A friend recently loaned me his Krell FPB 600c amp to try in my system (Meridian series 800 front end with Revel Salon speakers). I was impressed, especially by the startling clarity of the sound. What conclusions can I draw about other models of Krell amps based on my audition of the 600c? I understand that the "c" designation refers to the addition of Krell's CAST inputs to the FPB series. What does the added "x" designation mean? Does a FPB 600c or 700cx sound better than its smaller stereo brothers? Would a 350mcx mono pair sound better than a higher-power version stereo amp? How does the new EVO series compare to the older FPB, or FPB cx series?

I would greatly appreciate anyone who has some experience and expertise in the various generations of Krell amps commenting on whether there are notable sonic differences between the different Krell generations and models, and whether I would likely enjoy this or that Krell amp if I liked the 600c.
I have a 700cx and love it. I have had many Krell amps in the past including the 350mcs(450mcx) but the 700cx is the best I've heard in my system. Mine was upgraded from a 600c. I can't really tell if there is any difference in sound since the upgrade but it gave it 100 more watts per channel.
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??