Neither example provided in the article are correct in the context of marginal returns..
The first because the improvement is as a result of getting a dysfunctional system functional. "unlocks the previously unrealized potential of your other components"
Like replacing an incorrect rectifier tube or other similar build feature with the correct one. Fixing a flat tyre on a car. Everything will work better and as it was designed to. This is shifting the returns curve upwards, not a movement on it.
The second is changing the technology which adds a characteristic that wasn't there previously. Comparing two exact same cars but for the fact that one has a manual transmission and the other automatic.
Anyway, this is actually conceded - "Far from offering only incremental improvement"- but marginal analysis is only about increments. So, does not apply by the author's own admission..
I think its probably not a good idea to use the marginal return concept to audio equipment. It has a very narrow meaning, one that is exceptionally useful and informative in the correct context.