Is it worth upgrading a CD player?

Audiolab CDT6000 is very good. Being a dedicated transport, it would likely be a nice improvement over the old marantz cd player. 
But as others pointed out, DAC is at least as important as a transport. The CDT6000 just lays a pretty good foundation for a future dac upgrade. 

In my mind, CDs and streaming should always co-exist. I stream plenty via Tidal Hi-Fi Plus and other services (through a Bluesound NODE 2i), but most of my listening is spent with CDs (lots of jazz, but other genres, as well). A couple years ago I decided to upgrade with a CDP that would complement my Rega Elex-R integrated; I picked-up a used Rega Apollo CD player and haven’t looked back.

An important consideration about modern streaming is that when we stream a classic album, we’re usually hearing the record company’s latest remaster. That can often mean a louder, more distorted reissue when compared to the original and/or earlier remasters. Those earlier versions are rarely available to stream, having been replaced by the remaster of the moment.

Having collected CDs since 1985, I have many original issues, including many Japanese- and German-made discs, which mostly sound superior to remasters currently streaming. So to answer the original question, yes it’s absolutely worth upgrading a CD player!

+1 on getting a transport, if you are happy with your present DAC

Thanks again for the comments. I’ll look into which transports pair well with the Audiolab M-DAC+.