If the source and preamp are unable to deliver the goods there is nothing you can do about it downstream.On the other hand, if the power amp or speakers or room acoustics are limiting the resolution of the system, you can't do anything about it upstream.
I don't think there is a generally applicable answer to the question. It depends on the specific components, and which of them is the weakest link. Audiojan makes an excellent point, also.
Best regards,
-- Al