I think you will really be better off with a tubed DAC if your goal is to reduce listener fatigue caused by harsh upper mids and highs. But be careful. Unit selection and tube selection is very important to achieve this. Out of the box a tube unit can sound just as bright and fatiguing as a SS one. The essential difference is that you can change a tubed DAC's sound by simply changing the tube(s).
A recommendation - find a used California Audio Alpha or Sigma DAC and play with it and your Marantz. Be prepared to change tubes a bit. I have used both - the Sigma is cheaper and for all pratical purposes just as good as the Alpha. It only has one tube too. I presently have one set up with a Cal Delta transport in my Headphone system. I found that it sounds best with a JJ 12AX7 (an inexpensive new production tube). Compared to my Marantz SA11S3 this is warmer in tone and certainly not fatiguing at all. You could probably find the Sigma for sale on A-Gon, if you are patient, for $200, and better yet you could probably get it with a matching Delta transport for less than $500. And if you don't like it it should be easy to resell. They are still popular.
A recommendation - find a used California Audio Alpha or Sigma DAC and play with it and your Marantz. Be prepared to change tubes a bit. I have used both - the Sigma is cheaper and for all pratical purposes just as good as the Alpha. It only has one tube too. I presently have one set up with a Cal Delta transport in my Headphone system. I found that it sounds best with a JJ 12AX7 (an inexpensive new production tube). Compared to my Marantz SA11S3 this is warmer in tone and certainly not fatiguing at all. You could probably find the Sigma for sale on A-Gon, if you are patient, for $200, and better yet you could probably get it with a matching Delta transport for less than $500. And if you don't like it it should be easy to resell. They are still popular.