Better cd SQ

A solid Dac choice would be the smart next step. Without knowing exactly what the cirrus chip and output stage of the Marantz may render in tonality and stage characteristics, hard to say exactly what will differ.  It 2k for a dac would get you a jump in quality regardless! Cables do matter, and even digital outputs (and even between dac inputs) can differ in terms of results. But overall, something with a good balance of stage, clarity and spaciousness like a r2r dac would be great. Since you’re running tubes, however…delta sigma might be the right offset? Who knows. 

so many choices, but if you’re not particularly worried about digital input choices and are looking for something kinda special, for around $1700 the ANK DAC 2.1 Signature might be nice with your setup (tried posting a link but got blocked). I think that’s a kit? More to have it assembled. 

That PS audio recommendation would also be really nice with your setup (assuming here). But there are amazing offerings like the Gustard R26, Musician Audio Pegasus II, mentioning these as they are reviewed to hit well about their asking price in terms of tonal balance and performance. But like I said, it doesn’t have to be R2R to be engaging and beautiful, wide and deep while being true to source. I was lucky enough to find a used Okto Research Dac 8 stereo. Most balanced and beautiful dac I’ve ever heard and it’s a very carefully designed ess Sabre implementation. 
 

But good to consider a dac that you can later try with a decent transport since you enjoy the CD format. Yes, there are amazing new all in one’s that can attach an external drive, but I can respect (and prefer) the possibilities with separates. 

"No reason to waste money to play the CDs you have when you can have access to millions of albums at the same or better sound quality."

Such nonsense. 

Amour-all your cd's for a 20% improvement in dynamics....don't laugh.  Try it, you'll like it !

You’re at the bottom. there are many levels of upgrade from there. the first one is what you propose, an external DAC. It is easy to beat the onboard DAC. But to get a good audiophile DAC you’ll need a few grand. In the budget DAC area, $1k or so, look at Schiit. Then when you have an excellent DAC, you might want to iprove your CD transport. And of course you could spend money on a streamer (another several K) and subscribe to Qobuz and have CD quality by streamping with almost unlimited library.

Long term, the DAC is the heard to a digital system and you can keep upgrading it.  If you found $20K that you forgot you had, I would not criticize you for spending it all on a DAC.

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.