I suspect I am about to state that which you already know very well!
I think the centre is SO important in getting a first rate sound field using surround sound. So I would expect your centre to work synergistcally with your KEF mains; and be less effective with the other main speakers you list.
WRT the amp if money is secondary I would echo RackKit. That said I use a 300DR for my SBLs and a Nord 1 up NC500 for my centre and rears; this allows me to massage my guilt about leaving the 300 on 24/7!
In my case the speakers are: SBL; Axcess; and, Focal Domes as rears. I do have a 250 & 140 I could use for other speaker duties but I find the Nord to be excellent. Are the SBLs and Axcess fully congruous? No, there is a smidge of difference when vocals pan from extreme LR to centre, but this is rare and the voicing of the 300DR and CB amps is also not 100%.
I will watch your choices with interest.