Hi, Eduardo, that is an excellent question and one that probably has no right answer.
Synergy has a lot to do with the emotional impact of whatever you are listening to in addition to other factors. I have heard music concerts through a table radio that have raised goosebumps and they are an experience which will never be repeated again. At that moment there is a synergy between the listener and the music that transcends the equipment. And for me that is the most important kind of synergy possible.
In terms of the synergy of an equipment, there are many factors to consider.
First, and for me foremost, is the room where the equipment will be located. Many persons make the mistake of buying enormous speakers and superpowerful amplifiers and then place the equipment in a small room. Then, they experience no refined bass, too boomy, no soundstage, glare, etc. The room is part of the equipment. It influences speaker behavior and response. Thus it is the first element to consider.
Some people work from the speakers towards the sources. That is, after considering the room, the first thing they consider are the speakers. Then they work their way upstream: speaker cables, amp, interconnects, preamp, sources. Others start at the sources and work their way downstream. I have seen excellent systems done either way.
After all that work, comes the real test. The equipment must be able to recreate the original musical experience and make you part of it. If you can feel the emotion of the musical experience and at the same time be able to satisfy the usual audio parameters (soundstage, clarity, speed, pace, definition, etc.) you may be able to say that the result is much more than the mere sum of its parts. You have a synergistic system.
But, this is very difficult to achieve. That is why we all keep changing parts of the system frequently. And sometimes we discover that something we have just abandoned provided an element of synergy in the system and the new one doesn't.
Hope this will contribute in a small manner to enrich the dialogue.
Happy listening!