You might want to rethink your question. I doubt anyone would suggest adding a power amp, especially a tube one to a Marantz receiver. The pre-amp in the Marantz is OK, but cannot justify any kind of upgrade that you are thinking of. The Focals are easy to drive, so more power should not be an issue.
If you've got money burning a whole in your pocket, and I am assuming 2 channel only, why not upgrade to a good integrated amp, tube or otherwise?