Is it possible to really know what you're doing?

I do like the speaker and amp rolling. You could always get a equilizer to change the sound if your brain becomes fatigued to your sound. The body produces endorphins when it's happy like the runners high. It could just be from a physiology standpoint that we get use to the sound and don't get the runners high and it requires a change to get that back. Maybe we could get a gov grant to buy expensive equipment and study the physiology of addiction. Off to usaid I go. Enjoy the music and the hunt.

When I can ignore the equipment stack and enjoy the music. System synergy is the key and when I find it, I attribute it to luck.

Finding balance and perspective can hard for people who have the discretionary time and money for audio.

Sometimes when I find myself obsessing over audio, I look at other parts of my life and the world and suspect that I could be doing other things. Maybe even helping others. It helps restore my confidence that my audio is good enough. 

Our understanding and proficiency is increased when spend time with anything ... but do we ever know what's really going on?  No.

I think the OP is alluding to whether there is a holistic approach to system building, rather than trial and error. Sure there are some basics in terms of matching power/current needs from amp to speakers and some impedance/gain issues in the entire chain, but for me, I cannot look at specs on paper and conclude that a given set up is going to sound "real," leaving aside the room and set up of the system with a given room (the acoustics part, rather than the electronics/signal part). 

I built my current main system through trial and some minimal error- improving what needed to be changed to get it closer to my ideal. Some of those improvements might be regarded as "lateral" rather than "upgrades" based on price, others (like turntable isolation) were done of necessity, still others based on my experience from past set ups (electrical power).  I already knew a lot of what the core components did well and where they were lacking in this current room set up so had to learn how to take better advantage of a new room. Some of that experience told me what tubes I preferred in certain positions in voicing the system but these were refinements based on long experience with the particular gear in combination. The system is "tweaked" to that degree and a change in phono cartridge from a few other "high end" brands to another one "brought it home" that last iota--which was only revealed once everything else was in sync. 

This still leaves me with the conclusion that I have no universal approach to system building in the sense that I can take the abstract at a given budget for a particular room and apply a view or philosophy that will yield a predicted result with a given combination of unfamiliar components in a given room. Instead, it is listening in situ in a controlled environment that has enabled me to assemble and optimize what I had.

My vintage system, based largely on 50+ year old components which I have owned since they were new, is in some ways, more forgiving and I was replicating what I had in place in 1975 (with a few substitutions). But, though that system sounds great for what it is, it represents a "time capsule" system and not an attempt at state of the art reproduction (though it can be very convincing on certain program material).