Hi Hypnos 1,
Don't know about other people and their ears and hearing, but my personal experience is that it takes some time to come to a true conclusion of what sounds really good. First impressions are not always reliable.
In the world of racing cars there is a saying that if an engine loses five horsepower you notice it right away, but a gain of five horsepower may be much harder to notice. So translated into tubes, some glaring deficiency stands out right away, but small improvements may escape you for a while until it becomes settled in your mind. You may love listening to a set of tubes, but over time some grating quality emerges, and it could take time to identify it.
Then we have the ice cream syndrome: Strawberry, chocolate or vanilla? Everybody have their preferences, but if we can reach a consensus on which ice cream is best (6AH6, 6AU6, Heptodes etc) you can then safely pick your own flavor. And just as with ice cream the only real way of experiencing it is to taste it, so we have to try and see what suits us best.
Happy tube rolling!