the Luxman 507 with Harbeths are excellent choices. Not heard with the 509, and no experience with Mimas.
the tradeoff, as you are aware, is whether to get a separate dac/streamer you can replace later on or not. I'd consider the 507 + separate DAC as a better comparison between Lux and Mimas. Also consider the UI / Streaming options. If it has it's own OS/UI make sure you really like it and you trust the company to keep it updated.
There's a lot to be said for fewer cables though!