Better cd SQ

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.

I'm in the exact same boat as you, OP. I also have the CD6006. It sounds pretty good to me until I spin some vinyl on my budget setup. Relative to my vinyl, the CD6006 sounds a little grainy and veiled. I tried the Gustard R26 for a month and didn't feel it improved SQ to any great degree. The Pontus is an attractive option I'm looking at but at the same time, maybe the lesson is, I'm not a digital guy. 

I too have a 6006 used as transport initially to my PS Audio gain cell dac. Decided to upgrade from GCD got a ARC LS 25 and Pontus and was a nice improvement. I also stream and LP wit P6.