Remember the big craze for battery operated preamps and phono stages? They were popular because they operated in solitary confinement. Unplugged from the wall, their grounds isolated, their power quiet and non-interactive, battery powered units gave us isolation advantages not shared by AC powered devices.
We care about interactions between equipment because the greater the interplay, the greater the chaos.
What do I mean by chaos?
We have certain expectations of how our equipment should perform. They are usually formed in a vacuum–without consideration of how they will be connected to the outside world. When you choose an excellent sounding power amplifier, for example, do you consider how it might interact with the preamp feeding it, or the speakers it will be driving? I would think most people simply trust its design and, if one model or another has severe issues, those might well be part of public record. In other words, the exception, rather than the rule.
How adequate is your preamp/DAC at driving cables? Your cartridge susceptible to hum? Your turntable isolated from vibration, or power fluctuations?
Mentally placing a product in solitary confinement when making a purchasing decision is only natural. We all do it.
And when we talk about synergy between equipment, how much of that dialog has to do with interactions?
Let's look at synergy, or the lack of it, tomorrow.
