The "c" is an FPB 300 with CAST inputs The "cx" is what used to be a 200c but the new technology that Krell has developed allows 100 more watts per channel from their amps. Their smallest class A amp is now the 300cx. What was their 300 watt amp is now 400 watts. The 600c is now a 700cx. Owners of earlier models can have them updated and gain 100 watts per channel. Owners of earlier designs that have removable power cords can have their cake and eat it to by upgrading.