Synergy

Here's a definition:

"The interaction of two or more agents or forces so that their combined effect is greater than the sum of their individual effects."

That pretty much sums up what you are shooting for in a good sounding system to me.

Of course, with audio gear,the only thing objective are the specifications and any actual performance measurements you might have at your disposal. These provide the only metrics available for objectively determining synergy or no.

That's fine but we know that no set of measurements can tell the whole story behind what our ears are actually capable of hearing, right?

sme say that synergy has to deal with Impedance and gain compatibility.

how you do the Impedance matching
and how to do the Gain matching?

thanks

Mapman's definition of the term is the accurate one. Many use the term in a semantically incorrect way. There's really no way to predict or quantify if the sound/presentation of one's audio system will be, "greater" than the sum of it's parts/effects(other than the owner's subjective opinion). System or component, "compatibility" is what people actually seek. I suppose that, "synergy" sounds more upscale to some though. If one's collection of components achieves actual synergy(per it's definition); it's purely serendipitous.