In general, I think one may get ultimately better sound from a matched transport & DAC combo. However, to get better sound than many of the hi-end single box players, you really have to spend some megabucks.
One box players have gotten very good with matching the transport mechanism with the DAC inside the CDP.
As Uli stated, he had to spend two times the amount of money of the Wadia 861 (which is a very nice player) to get better sound. I have an Ayre CX-7 ($3k retail) and have been told that to get objectionably better sond, I would need to move up to the Wadia 861 (at around $8k or so retail). I can only imagine the kind of sound that Uli is getting.
Unless you have megabucks to spend, I would stay with a all in one box player.
KF