The trick I think to upgrading is understanding what you want or expect out of an upgrade sound wise. Balance is also important and can easily get lost in the shuffle if one focuses too much on any one element at the expense of the whole, example- soundstage or ultimate detail or dynamics. These can all exist in one system but takes a lot of careful thought and matching and not a small amount of $$$. Get comfortable with your system as it is and try replacing (or demoing) one piece or equipment change at a time to ensure you like the difference it makes without overly impacting the things you already liked. If you really dig the 70's Marantz sound, you may try a foray into the McIntosh world.
Is upgrading worth it?
I know I'm probably asking the wrong folks. I'm sure most people would upgrade their system any chance they got but at what price? I noticed the more I try to improve my system the less my music collection become listenable. Higher resolution? So much of the music I enjoyed growing up sounds more listenable on my 1970's Marantz receiver and Advent speakers so anything I do now seems like a step backwards. I need to have two systems, one for high quality audiophile approved music and a system for all the rest. Does anyone else feel this way?