another_jim wrote:One variable at a time may work for learners, but as an operating guideline for experienced espresso pullers it is both wrong and stultifying. .

I'm seeing a pattern here over two years, and it's beginning to look a lot like (like I suggested in my first post with this question) noobs should stick to one variable at a time 'cause they are tuning their technique as often as tuning in a coffee. Once you are familiar with how the variables interact with one another and are interdependent of one another, make the changes that you already know are needed and then adjust from that new 'starting' point.
In a nutshell, the answer is 'it depends'. ; >
Like it so often is!

Greatest difficulty I find in tuning my own senses how good espresso should actually taste. I do not totally trust my own judgment, and to say merely "if I like it, then it's good" is simply not enough.

Drinking coffee is very seldom solitary activity, and it is important to me that also people to whom I offer a drink will enjoy it as well. There is the governing variable (from my perspective anyway). Creating "standard" espresso is a good start, whilst catering to palate people around you comes later, I gather.

One variable or multi-variable analysis is a fun and makes one to grow to better things, but if the target is off, than the effort might be possibly misguided. I do run around a city I live in, trying to find a barista, who has an international experience, have been exposed to good environment, and received enough feedback to know how to gauge his palate, and ultimately can offer me espresso I can aspire to. Unfortunately those people do not talk, and unless you have some special relationship with them, it's hard to get good benchmark.