Krell FPB from C to CX

How big is the difference between the latter CX family and the older C modell?
Soundwise and technically!
Is there a differnce in tonal balance, darker, more open, sweeter hights aso.?
I notice weight is thesame!
Is it via the cast connection most improvement is to be won, or equally as much through xlr/balanced?

Could the upgrade be done by retailer techs or, do i have to send the amp to Krell in the US?
Also wish to know, if Krell have some kind of service center inside the EU-region?

Thanks!