Some components too old now?

I dont know those specific components well, but in general i agree with Eric and others.  The preamp is probably fine the DAC likely can be vastly bettered ( and Schiit is a great place to start). I can't speak for the Marantz, but bear in mind that SPDIF is NOT, repeat NOT, a purely digital signal - no time to go into why, by you need a low jitter output.
A new DAC with USB and/or network in also lets you move toward the future, which is streaming.
I am constantly amazed that stuff that was great 30 years ago is often still great. But not digital.
G

Oh, one interesting caveat.  We had a group listn to two old DACs - that were eye -popping whne they came out 20-25 years ago, and one new one, a schiit BiFrost Multibit.  Long story short the 1994 Theta DSPro hung on with the Schiit - and was generally a little weightier.  Still a top notch, musical product. Note Mike designed both.
G

I’m using an optical connection between the CD and Aragon DAC. I must have discovered long ago that this gave me the best sound. Is USB any better...?

PS I don’t think I’ll ever use streaming or HDD so that’s not a consideration. I’m primarily vinyl, and CDs are so cheap these days second hand.

Miss my Sentec Diana DAC, looked for it for over a year, used it to my Quads -63 (still in the wardrobe), must be somewhere...

@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