Benefits of brand synergy

I don't know about Naim, but usually good digital will work just fine with any brand decent electronics. As for preamp/amp, it is an interesting question which I believe cannot be answered in general terms, only on case by case basis. And also it might not be better or worse but just different. Within my price ranges I follow the path of integrated amps and don't bother about that.
Yeah, many mix brands to tailor the sound as precisely as they can to their taste. Given sufficient funds, time and patience, I would too.

I would match power amp, speaker cables and speakers first, and the rest "doesn't matter", in a manner of speaking. But if we are talking reference level components, let's say, Gryphon, Lamm, Rowland, I would certainly try all possible preamp/amp combinations and would likely like them all. But they would sound different, I might even prefer different combination for different music, like different cartridges you know.
Of the brands that I just mentioned my first instinctual move would be to try Gryphon preamp with Lamm power amp and Rowland preamp/amp combo. Don't ask me why, I don't know.