Thanks for responses. Some logical thoughts here.
I ran a CD lens cleaner. Not a lot of difference, maybe a little improvement.
The Marantz (6004) will sometimes struggle to read CDs if they are not pristine.
Doesn't happen very often at all, but more often than with a Sony (non-ES), first gen-Denon from 1980s, and a Luxman I previously had.
But it usually skips/skitters if that is the case. It is playing this one flawlessly as far as going through the tracks with no skipping. The sound quality is just poor.
The first post re too much input gain during recording is starting to make sense.
Inspecting the speakers, I removed the grills, which hold in place with magnets, and found one magnet had worked loose and attached itself to the tweeter on the right channel speaker. Thought that might be a culprit but did not seem to make much difference.
I may sell the CD or take it to work, try vinyl if I really miss it. My vinyl playback is the heart of my system, a VPI Classic, Sumiko Blackbird LO MC run through a Parasound JC 3 pre. The old Yamaha integrated (run through line to the JC3) punches above its weight and I've had it 35 years or so. The Marantz is good about some things. I've never had to have it serviced, and some CDs sound quite good on it. I listen to vinyl most of the time.
All that said, I do have some investment in CDs, and want to make sure this is limited to one recording, not something in the CD player or speakers.