Actually there is considerable difference between a CD transport (what you are using your Marantz as) and dedicated transports. I have and use regularly an Audiolab 6000. A transport on devise. I have 3 very good CDP’s - used as a transport alone the Audiolab is ’better’ than any of them, however when used as a CDP they sound much better. If you are happy with your present DAC I would simply upgrade your ’transport’ to something like the Audiolab. Cambridge has a pretty good one as well. Personally I would investigate getting a good DAC and Transport.
To answer your basic question about the level of improvement I would say this depends on your level of listening skills and the importance to you of the differences you might hear in new stuff. If you are presently happy with what you have, why change, just continue to enjoy your music.