I will be the contrarian here and suggest that your time of listening to an old NHT/Rotel system AND through headphones (especially) has thrown off your perception of what sounds natural/real. I think you should recalibrate your ears to what real acoustic instruments sound like before making any changes at all. Of course, this really only applies if you are trying to get real, natural sound from your setup. If all you want is to get more exciting sound than what you have, then have a blast changing whatever you want that meets that subjective requirement.
Good luck,
Mark
Good luck,
Mark