If you don't mind me saying so, I think the example you provided above is rather inapplicable. I don't mean to insult your DIY skills in particular, however if a DIYer were to use components of the wrong value altogther is building a poor product and doesn't know what he/she's doing.
The assumption is that a commercial business do actually know what they're doing and are using the right parts (or at least the right values at the very least, but maybe not the best quality components from a cost perspective).
My belief is up to a certain point of a product design, it can be deemed as "right" and "wrong" design. But beyond that point, is a matter of design style preferences, management of manufacturing costs (parts & labour), etc. And in commercial products I think the differences we hear in the SQ are more due to the latter preferences - and that's where the resulting variance can be deemed "minor" depending on the listener's ears.
Using components of the wrong values is like a DIYer putting square vs round tires on a car and saying that it makes the most difference to the performance of a car. The car manufacturers hopefully wouldn't consider putting square tires on all their cars in the first place.
Sean you are right here, manufacturers (most anyway) use the appropriate values. But sometimes for cost reasons they might use a mediocre part but still with the correct values.
No capacitors and resistors are not the same thing. But both if them influence the sound. Take for example the diymod, the appropriate capacitor value should be calculated by :
where f is the -3db roll off frequency you want to achieve, r is the load (headphone + any inline series resistor) and c is the capacitor value.
using the correct value of c is important as a low value will have no (less) bass. Too high a value while it might sound good, there are distortions associated with capacitors(another formula) and also because the capacitor is not charged appropriately all the time, the chemical process in them will deteriorate faster and thus the cap will fail.