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.