Benefits of brand synergy

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I bought two pairs of Pass Labs X-600 monoblocs and a Pass Labs X-02 preamp. The sound was very good. I inserted an Audio Research Ref2 MkII instead of the Pass preamp and got immediate improvement. I again inserted an Audio Research Reference 3 preamp into the system, it was head and shoulders above the Pass Labs preamp. Both preamps were $10k at the time and both were flagship preamps of their company at the time. For me, this blew a huge hole in the brand symmetry theory. Brand symmetry sounds like a good idea, but I still believe that you have to listen and audition multiple components to see what works for you.
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"Brand Synergy" = "House sound" and that hasn't worked for me. In general, I like ARC amps and CJ pre's, and now I own neither in my listening room; but in order to get the sound that I like, as opposed to what they think is best, separate brands is the way for me.

Looks like responses are all over the place. No surprise here. It's an individual thing and no right or wrong answer. For me personally, amp & pre amp same brand. Rest doesn't matter.

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.

"Don't know too much about Naim, but is it possible to use Naim with non-Naim?"

Yes it is. It used to be and still can be a pain in the rear end. For the longest time, Naim only used DIN connections (except for turntable inputs). They now have DINs alongside RCAs. However, the DINs still sound better, going DIN to DIN from source to preamp, and pre to power. I'm not sure if their power amps have RCA inputs though.

Their preamps must be run by either a Naim power amp or Naim external power supply, as their preamps don't have an internal power supply.

Naim sounds best with Naim IMO, due to the DIN connections and star grounding (basically everything gets grounded through the source).

I've heard Naim amplification with non-Naim sources and vice versa, and it almost always sounded off for some reason. The sum is always greater than the parts to my ears. The only non-Naim digital sources I've consistantly heard sound great with Naim amplification is Rega. The Linn LP12 sounds right at home with Naim stuff too.

I've only heard 2 Naim speakers I'd ever own - the Ariva and the current Ovators. Wouldn't pair them with anything other than Naim though. I'm a big fan of Naim, but can't get passed their speakers. Too dry and wall of sound to my ears. I've wanted to like a few pairs, as they do a lot of things quite well, but I just can't listen to them for more than a half hour before I'm looking for something else.

I own a Bryston B60, as I think it does the PRaT stuff just as well, and adds body, detail, imaging and soundstaging. The best of all worlds IMO.