I think that this may be as much to do with the human factor in the audio world. I have experienced this personally with some tubes that others say sound different, yet I cannot hear the same difference. It is all so subjective; we all hear things so differently. I think that sometimes when we make changes to our equipment, we have some expectations of how much audible difference we should hear, which are not always based on sufficient understanding of the change or mod we are making. Sometimes the changes are extremely subtle. I would say that most of my LD1+ mods were subtle, yet collectively they are very noticeable when compared to my un-modded LD1+ that I use as a reference.
I built a new DAC recently and changed the (very good) op-amps for Burson descrete amps, if there is a difference, it is very subtle, to my ears. Yet some of the other changes I made were like night and day.
I suppose, in the end it is all about finding a sound yuo like and sticking with it, unless, like me, you are addicted to trying to make something as good as it can be (subjectively, of course).