Hi Dev,
Some good points have been made by the others, but I'll add the following thoughts.
Let's first recognize, as you do, that high quality speakers and electronics will usually come from different companies, for obvious reasons beginning with the fact that the technologies are very different.
Then add to that the fact that manufacturers of high quality amplifiers and preamplifiers tend, in general, to either not produce source components as well, or to produce a limited selection of them, constrained to be within a certain price range and feature set (which may not be what the buyer wants).
So for the most part the question narrows down to synergy between preamp and power amp. But then consider that the most important synergistic interaction between the power amp and other components is likely to be with the speakers. While the most important synergistic interaction between the preamp and other components could very well be with one or more of the source components, especially where a phono cartridge is a source.
So as I see it the possible benefit in synergy between preamp and power amp, that might result from having both components manufactured by the same company, will tend to be overshadowed by the different synergies that are required between power amp and speakers (not to mention the room as well), and between preamp and source components. Those being among the reasons that we often see solid state preamps being mixed with tube power amps, and vice versa.
Also, I'll add to the comment that was made about technical specifications that I would draw a distinction between the terms "synergy" and "compatibility." I would use the term "compatibility" to encompass avoidance of gain mismatches, impedance mismatches, mismatch of amplifier power and speaker efficiency, etc., things that can be determined based on technical specifications. Essentially, the ability of the components to function together. "Synergy," on the other hand, to me (and I think to you and to most of us) means that the sonic performance of the components is complementary, such that they reinforce each other's strengths and/or mitigate each other's weaknesses.
Regards,
-- Al