"System synergy" = What sounds best to your ear, and it is subjective

Now, allow me to use an analogy here, I don't claim to be an expert in audio science, so don't attack me for over-simplifying things:
Each of us has different ears, and the sound that we like can be expressed in a frequency response curve (from 0kHz - 20kHz). (ok, I'm oversimplying things)
Now, every piece of equipment in our system (CDP, preamp, cable, amp, speaker, room) does not put out a flat response. e.g. Tube amps tends to have a little bit of "bump" in the midrange than solid-state amps. e.g. Cables tend to act like filters.
When we put all these in a chain, what we get is a superimpose of different response curves, and then our ears serves as the final filter before it gets into our brain. Let's call what gets into our brain the "target curve".
The point is, different people like different target curves, and there're more than one way to achieve that target. So "system synergy" simply means you have find the right mix of equipments and room treatment to provide you with the target curve that sound best to your ear. At the mean time, others may not care for your target curve at all.