There's a wide gulf between an all tube integrated such as a PrimaLuna and an SS Denon receiver. There are very good SS amps that aren't terribly expensive that would do the KEFs justice. Examples are the Odyssey Khartago or Stratos, a pre-owned Bryston, or new Parasound, whether from the Classic or Halo product lines.
Given that you really like the KEF R500s, I think you should get them. You could get a multi-channel AV surround preamp/processor (NOT a receiver) and a Parasound 2-channel amp for the KEFs. The pre/pro would also have a subwoofer output and you could get a powered sub to flesh out the bottom octave of bass. You'll want that for home theater anyway.
Then when you're ready to go multi-channel, just buy the amplifier(s) you need for center and surrounds and you're good to go.
You could also shop for an integrated that has the inputs/outputs to provide home theater bypass. This enables you to use the premium amp for two channel listening and integrate it into the multi-channel setup (under control of the multichannel pre/pro) for 5.1 or 7.1 home theater.
Given that you really like the KEF R500s, I think you should get them. You could get a multi-channel AV surround preamp/processor (NOT a receiver) and a Parasound 2-channel amp for the KEFs. The pre/pro would also have a subwoofer output and you could get a powered sub to flesh out the bottom octave of bass. You'll want that for home theater anyway.
Then when you're ready to go multi-channel, just buy the amplifier(s) you need for center and surrounds and you're good to go.
You could also shop for an integrated that has the inputs/outputs to provide home theater bypass. This enables you to use the premium amp for two channel listening and integrate it into the multi-channel setup (under control of the multichannel pre/pro) for 5.1 or 7.1 home theater.