Imho you must choose a very good dac to beat the internal dacs the 6000 OSE has...

and yes, it has more than one.
The serie 6000 from Marantz cdps use two SM5872BS DACs in a differential configuration.
Dual differential DAC implementation offer quite a big improvement over a single-ended configuration, especially if the entire design path is complimentary, which is the case here.
The models 6000 maintain Marantz's tradition of having separate active low pass filter (LPF) stages from dedicated output stages. Model 6000 uses the well known and traditional to Marantz op amp, NJM2114D, for the LPF filter stage and Marantz’s proprietary HDAM discrete modules for the output stage, while model 6000 OSE eschews the NJM op amp for another pair of HDAM modules, uses better quality caps and better transformer.
One more thing. If you choose the dac path and goes optical, imo you should try to find a away of experiment the many optical cables possible. They all sound different...
Btw, we at ST have something about dac's and digital stuff here :
http://soundt.18.forumer.com/index.php?showtopic=787&st=15
Enjoy your new toy.