Being 78 and having age related hearing issues and not knowing what areas of what you’re hearing you need to improve, I’d take most responses here with a grain of salt. You’re going to get lots of responses that are wide of the mark.
I have the Technics SU-G700M2 and trust me when I say to ignore any responses to replace it. What I’d like to know is, are you using the RCA out on the Marantz CDP or one of the digital outs? I ask this as the Technics is a fully digital DAC amp and you’d need to spend a lot on a DAC to improve on what it can do for a digital signal.
Saying that and assuming you’re not using the RCA out on the Marantz, try using a great Toslink optical cable (mine is from DH Labs-click on the blue link) as it bested any RCA cable or Coax I have on hand. I haven’t tried many but the sound I’m getting is marvelous.
The Marantz should be good enough to serve as a transport and if you like what you hear, you can invest further down the road on a better transport when and if you feel like it. Also, check out Darwin Cables and their Specials page for cables used at audio show as you can get some great discounts on some great cables. Hope that helps.
All the best,
Nonoise