Is it worth upgrading a CD player?

Until I upgraded to an Audio Research Ref CD9 I had no real appreciation of the influence of source, internal DAC, tube output stage, etc has on the performance of a CD player. This unit is not the DAC of choice for all my digital listening. And this fixed my perception of harsh leading and trailing edges from the previous Esoteric K03Xs CD player.

So my answer is certainly yes.

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 Those earlier versions are rarely available to stream, having been replaced by the remaster of the moment.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