As a computer hardware developer I can categorically say, that the statement "USB audio can never sound as good as CD" is incorrect.
CD Audio source is 2 Channel signed 16-bit Linear PCM. This means that the hardware in your CD player does not even need to cope with data rates exceeding 150KBps (1xCD speed).
The main points to understand are:
1. Hardware limitation.
2. Audio encoding limitations.
Hardware Limitation - CD player use motors, and often power supplies mounted near their preamp stage, which while I cannot detect definitely effects the audio signal due to magnetic fields of both the power supply and the motor used to spin the CD.
The DAC (digital to audio convertor) as well as the clock will have an impact on the quality of the sound any digital device has. Both computers, and CD-Players have these. The clock keeps the timing of the music accurate while the DAC uses various algorithms to covert the digital source back into an analogue signal for the amplifier/speakers.
Clocks are generally very accurate these days, loosing or gaining a single second over a month or more. So audibly you will probably not notice the clock even if it is slightly off.
However the DAC is something you will notice. Each DAC chip will boost the analogue output differently, they also use different algorithms to convert the digital signal back to an analogue signal. Some of these DAC can also up sample the digital signal, and have present equalizers. Typically the manufacture will not tell you the algorithm used or if they up sample the signal or not.
Encoding Limitation - Due to the fact the encoding used on CD Audio disks is relatively low quality by today’s standards. Your CD is inferior to the medium it was mastered on, usually Hi-Resolution Audio files on a computer. Which are easily transferred between computers and DAC over USB, PCI, or any other interconnect. Also due to the limitations of the encoding used for CD Audio disks, it also means that computers can accurately (100% accurately) copy the file and play it back on a DAC of your choosing.
Codecs, sample and bit rates combine to give a natural or unnatural sound in the digital world. Listening to the same song sampled at 24bits@96Khz and then at 16bits@44.1Khz, and most people can tell the difference. There is then the codec itself, some codecs partially overlap the left and right channels to save on storage space, namely MP3. This leads to less stereo separation on high-end speakers, but is not usually detectable on budget and midrange speakers. If you opt for lossless compression and dedicated right and left channel streams you will have better stereo imaging.
USB Controlers - USB controllers can communicate with devices using different modes, bulk, async, sync, adaptive and so one. If you are not using a HUB between the controller and the device the mode is mostly irrelevant as there should be enough bandwidth to stream Multi Channel HD Audio even on USB v1.1. USB v1 had a maximum transfer rate of 12MBp/s, which far exceeds the requirements of anything recorded on a CD. Thus only USB controller with too many devices attached would degrade the sound supplied by 16bit @ 44.1Khz sampling.
USB Cables - A damaged USB cable or failing that, a USB cable which is too long would also degrade the signal enough to cause issues with sound. However as long as the cable quality is matched to the controller you should be okay. Thus an USBv2 controller should use an USBv2 cable. The only other factor that would cause the signal to degrade on the USB cable is strong external magnetic influence.
Conclusion
For 90% of the population who do not have lots of wires/magnetic influences next to their USB cables, the introduction of USB into an audio signal path is irrelevant. What is relevant is the clock in the source, and the DAC used.
I for instance use a M-DAC+ as my DAC with various sources including a CD player over Digital Coax, Mac over USB, and AirPlay over Optical as I prefer this DAC.
And while I prefer to purchase Audio CD’s rather than use High Resolution Streaming Services, this is simply because I choose to own my music and not rent it. It’s the same reason I own my car, and my house. I could rent a nicer car and a bigger house but when I retire and can’t afford the rent payments anymore where will that leave me, Homeless, without a car and with no music that’s where!
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