+ another one @charles1dad
and +1 @kota1 "I’m in the stop adding boxes camp..."
But overall: If you like your Marantz, and it’s running like a champ... well, in my opinion roll w it. I mean, how much do you want to spend and how many boxes do you want, or is the M good as is for you? If you do get any sound quality upgrade with some other option, will it be THAT much better, really, honestly, that you would sit up and take notice?
I’m listening to an old (2004) Sony cd changer as we speak, and it sounds excellent, though admittedly - and I was surprised, actually - my single-cd Marantz Ruby does sound a bit better (but it better, at $4000 list, though I got mine open box but still $2400).... but not by a wide margin, and I have to listen for it; nobody other than a concentrating discerning audiophile would ever notice, at least not consciously. I only got the Ruby cd player because I couldn’t pass it up to complete my Ruby system when I bought the amp at the same time also open box price, and it does provide a great dac for the streamer etc too built in w/o adding even more boxes... I’m glad I did, but it’s an extravagance for my simple practical tastes.
Now, the Ruby also has a great dac built in for other component to input through, so I have my Bluesound streamer going into the Ruby dac via coax, and I could put my Sony cd changer for those days when I want to spin five discs in the background into the dac of the Ruby via optical. Currently I run my TV into the optical of the Ruby cd player, though if I get a basic Schitt for the TV then I could I suppose put the Sony player into the optical in of the Ruby... but I’m not in a rush, because the Sony with its own internal dac does sound great just right into the back of the amp via RCA. ... and I suspect that might be the case with your existent Marantz, as well.
But that’s my system, and I’m absolutely not willing to chase after some ultimate dac beyond it, because now you’re onto a whole other hunt, obsession, expense, debate and so on.