Only thing I would add to what you said Conrad is that trusting your ears has to be taken with a pinch of salt in some circumstances. In a shop, comparing two amps, speakers or pedals is fine. The differences would be significant and, with quick changes, your ears will tell you which you prefer. But your ears have a limited ability to discern true differences when they are more subtle, (no matter what some "golden eared" people think). They have a very limited ability to hold an accurate impression for longer than a few seconds, so for example replacing components other than plugins in an amp is not really reliable, the change takes too long. And worst of all, they are often influenced by your mental state, as in "I WANT there to be a difference so, there you go, I can hear a clear improvement". The legendary "Placebo Effect".
The only way your ears can be really relied on is when they are making a quick comparison of sounds and when they don't know which they are hearing. Step in the Blind Listening Test. This approach has been instrumental in making me the sceptic that I am of the improvements offered by most demonstrations of "mojo" in music. I was first introduced to it when I worked in the hifi industry where it is absolutely essential in design and development, and I've applied it since then to some areas of guitar work too. It showed me that there is no one that I have yet met who can tell the difference between many of the components that are quoted as "critical" when applied to guitar.
For example, there was a huge outcry a few years back when it was discovered that Gibson were using plain old ceramic capacitors in the tone circuitry of many of their guitars. At their prices, why didn't they use a premium capacitor type like the fabled "orange drop" which is famed for its golden sound? Well the answer is not that Gibson the ultra huge corporation were involved in ripping off their buyers, it was simply that Gibson could hear no difference between the two and, rather than spend £1 when 1p was just as good, they did the sensible thing as any large scale production line manufacturer should. Now I would have to ask, if these caps made the guitars sound so rough, why was anyone buying Gibson guitars in the first place? The answer to that one is that the ceramic cap usage wasn't discovered by using ears and hearing a poor sound, it was discovered when someone opened up the back, saw what was in there, and raised a hue and cry against Gibson "'cause everybody KNOWS that the premium caps sound better". And so the witch hunt started. Up until then there was no problem! After that, no matter how much Gibson may insist there was no difference, the "mojo gurus" continued to poison the well. (And a lot of them made a lot of money selling completely unnecessary expensive caps to boot!)
In that case, (and the case of most magic cap types in amps too), setting things up so cap types can be swapped instantaneously with a simple switch inside the same guitar, and then conducting blind tests with those who reckon they can tell the difference doing the playing and reporting, guess what it shows! And another area is diodes/bipolar transistors in distortion pedals. There will be a difference between germanium/silicon types but for the rest of it, they're all effectively doing the same job in the same way and differences will be absolutely minimal and generally indistinguishable in the context of what they are doing. With modern circuit topologies, (WAY better than the primitive approaches of the 60s), it's the chosen circuit topology and eq which can make a pedal sound different not the semiconductor types. PCB use in guitars is slated without any proof it makes any difference, (it doesn't change the sound at all, it actually offers benefits in other areas), and many techs offering hand wiring refits are raking it in!
You can fool all of the people some of the time, you can fool some of the people all of the time, but you can't fool anyone once they decide not to be fooled!