Memo, your post could be construed as a "bits are bits", source makes no difference. This is absolutely not the case. A cd player based system is usually synchronous, while pretty much all USB dacs are asynchronous. In practice, sometime fairly dramatic differences in sound quality are possible between the two, mainly because of differences between jitter levels of whatever device provides the master clock (which may be the cd player, a usb converter or the dac) in the architecture.