I choose to take this post personally because I literally just ranted about how much I hate the whole "synergy" notion in the context of audio, just a few posts above.

Jokes aside:
The textbook definition of synergy is pretty much "two or more components interacting to produce a combined effect greater than the sum of its parts".
And as such, it's utter nonsense in the audio world. Components do not need to be "synergistic"; all they need to do is their job, and do it well. If they do, then the whole system will work just nicely. There's even standards for ensuring that, for example output and input impedances of preamps and poweramps, to ensure any combination of devices works fine. In case of power amp - speaker interaction, with the most "wildcard" component being the speaker with its complex load, there has been a decades old trend to simply make the amp as stable and load independent as possible, to fix that problem. And it works. Both beefy AB and class D amps that simply do not care about the speakers' behaviour - within reason - have been available for a very long time, for affordable moneys.
"Synergy" frankly is a bullshit word and meaningless in this context, and I really wish people would stop using it. Talk about competent devices instead, that makes a lot more sense.