It will be hard to find an amp to tame the highs in your speaker. I know you don’t want to hear this, but trying to tame the speakers is a band aid approach. I would cut my losses and put the speakers up for sale. Then listen to different speakers and find one you really like with smooth highs. They should then sound fine with your Marantz. I speak from experience as I had a pair of bright speakers and changed all the components in the system including cables. After wasting thousands upon thousands of dollars, I came to the realization to change the speakers. Problem solved.