In the last 12 years I owned several CD Systems, mainly separate ones, Transports from Goldmund, dCS, CEC, Wadia...with separate Dacs.
At that time I thought that is the way to go but when I made a comparison and something was better from sonics, the Transport was not able to rescue my Digital Hardware.
I think, digital quality is mainly based on the internal processors, the brain inside and how it was done. I sold all these units some time ago and went to a 1-Box Design from EMM Labs and it good enough that I can listen to it for some time. In a way for me the best Digital decision I made all over the years.
The real secret in enjoying Digital Playback is the Mastering of the Disk you like. There are the most differences, a good 44.1 can give you more pleasure than the same in transferred SACD (or HDCD...). No transport can improve a weak Master ( using Laser clocking and so on in the process...CD was a mass market product and in the last years it became cheap and worse...) to a good sounding one.