I have an Oppo 105 which I've enjoyed for years. Highly regarded and lauded. Then Iauded a Schiit Gungnir DAC, making the Oppo a transport and the sound just bloomed!
Your Marantz first was produced in 2007, 15 years ago. You could probably stand to invest in a more modern CD transport - the California Audio Labs Tercet Mk 3 comes to mind and then experiment with DACs.