Some components too old now?

Technology has improved by leaps and bounds in those 30 years.
If you can use the Marantz as a transport, I would invest in a good DAC like an Ayre Codex or Schiit Gungnir or Yggy. Not too much money, but lots of quality performance. And, they offer updates when available.
Also, I wouldn't spend more than $2K for a DAC, as they have been improving so quickly, and that $2K can get you something that will sound as good as something more expensive.
IMHO
Bob

Like you said, it was one of the best in its' day, but that was more than 20 years ago. I think you'll find just about any DAC built recently will be a significant improvement.
I found a link to another forum (hopefully allowed) asking the same question:
https://www.audiosciencereview.com/forum/index.php?threads/dacs-old-vs-new.3229/
B

@vinyloid,
Yes, I got the same take on those thread. So, maybe I'm wrong. The only way to tell is try and audition some more contemporary DAC's. Though I have to say DAC's from the 90's never contemplated the high resolution being offered today.
Re USB, a touchy subject...
For me, I loathe USB. Way too twitchy and dependent upon all the equipment around it. Just look at all the products offered to help clean up the USB signal.
I prefer optical. You just can't pinch it -(like Verizon did. It took 9 months for a tech to find the problem of my services dropping erratically).
Bob