I've owned quite a few DACS and CD players over the years and I now have a state of the art setup (Jay's CD3 Mk III, Berkeley Alpha Reference Series II MQA DAC). My system sounds better than it ever has but the addition of a single digital component has never been a big leap in sound quality.
My advice is to buy a well regarded used DAC and see if you can hear an improvement. Once nice thing about adding a DAC to a CD player is that you can easily compare the sound of the Marantz DAC with the new one. Some people claim to hear a huge difference in DACs and you may be in that category. Many blind tests, however, have shown that when the listener doesn't know which DAC they are listening to, they can't tell them apart. If you want to climb on the DAC treadmill then buying used is a good way to go because you don't lose a lot of money if you upgrade your way to finding the holy grail.