Benefits of brand synergy

Many good answers above. On the basis of sound quality alone it depends on if you like the sound. Of all the one brand systems I have heard there were not many that were outstanding. It is very difficult for any one manufacturer to design one very good component let alone several. I prefer to mix and match. It is more difficult and time consuming but can yeild superior results. It can also be frustrating some times.

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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.