About synergy?

The definition of synergy is that the sum is greater than the individual parts. The sum is undefined, making synergy something of an irrelevant term in audio.

That being the case, and more relevant, is each person tries to build a system that is pleasing to them. Similarly, each piece of equipment is designed with some sonic signature that is pleasing to the designer, given nothing is perfect. Of course, interactions among components come into play, which is beyond the control of individual designers. So you have a choice, either find a designer/brand that matches your tastes, or mix and match until you come up with something you like.

Again, the sum, or perfection is undefinable, therefore different for each person. So synergy as you have defined it really has no useful meaning.

If you are talking about mixing and matching to compensate for unappealing designs, I'm curious why someone would do such a thing. Why not just pick components you actually like, following basic principles like impedance matching, efficiency etc?

There are so many variables in audio all interacting at the same time (including the interaction of the acoustic space (which can negate synergy of same brand mentality), that its a bit like a multitude of drugs interacting within the body ( substitute the body for the acoustic space). That's why high end audio is a crap shoot, an endless fascination/frustration that keeps most of us going in this hobby.

I'm not clear on the question , but if the OP is asking if same brand same sonic signature is a way to obtain audio nirvana , the answer is likely no for most of us . If a preamp has a warm sonic signature than the last thing we need is a same brand power amp that is also on the warm side . Two warms don't make a right .
I have owned matching electronic's from Levinson , Audio Research and Ayre , all great stuff but all imparted there own house sound on the music . A variation with one major component can make it very difficult to hear that all to distinctive sonic signature . And it's never a crap shoot unless you purchase equipment without hearing it at length in your system .

Around these parts we use synergy to describe what you have, as all pieces in a system working together for a harmonious fit. Such as a speaker and amp working together (synergy) or that combo working well in a particular room ( synergy) or a cartridge working well with a particular phono stage... Etc...
Once you're aware of such systems, you'll know from then on if something is working synergistically or not. Well, some people seem to catch on!