I would change the Marantz PM6004 for the Rotel RA-11 amp and connect the tuner and CD player digitally to use the Rotel's inbuilt DAC. That will be far cheaper than buying an external DAC.
I don't know if you are able to arrange a demo and listen to these items first, but I doubt it will make much difference. The Marantz amp is very good but it is a £250 amp. To get a big improvement over the Denon tuner and Marantz CD player you will need to spend quite a bit more than that. There is little point spending a lot of money on a DAC just to feed a £250 amp, no matter how good the PM6004. If you can arrange a demo and you hear a value for money improvement, then great.