Marantz's clocks on the CD63 and CD67 series players were pretty appalling (~700ps jitter, giving max resolution of around 13.5 bits).
I don't know how the CD6000 compares in this respect. However, it's not a brilliant player to begin modding with because the transport mech isn't anything special, the internals aren't hugely generous on space, etc. For the same money that you could start modding up the 6000 for you could get a better starting point player (Pioneer stable platter, battleship marantz (eg CD10, CD16), Teac VRDS, Sony fixed-pickup) and then mod that as funds allow. Generally these would make good transports, so you could try running them into a DAC... NOSdac and SuperDAC are popular.