The Kef speakers are relatively neutral while the Pioneer is rated by manner to be bright. THe Yamaha too is said to be brigher and more clinical than say a DEnon or Marantz receiver, but not as clinical as Pioneer are often suggested to be. Clinical receivers can be very detailed, but also fatiguing to listen to for long periods so I'd maybe go with the less clinical Yamaha receiver. It is ultimately up to you though so I'm at a loss as to why you don't simply try them for yourself if you allready own them?
There's no wrong or right combination. THe one that sounds best to you is the one you should use.