Beware making changes in well designed gear.
I have seen both John Curl and David Berning deplore such 'improvements, saying that if they had wanted more expensive components in a given circuit they would have put them there - they designed for sound, not price.
And if you are talking about as market gear, I doubt that any substitution will make much improvement compared to buying a well designed piece of gear, even if it turns out to be pricy. (Sadly, there is lots of pricy gear that fails to give good sound too.....)