Differences between cd transports?

Hey Reb, hope all is well.

Many things might account for one digital source sounding different than another. Jitter is probably the most common culprit. Some DACs can correct for that, others not.

Assuming transports are working properly, there is also often errors in reading optical discs in real time as CD players must do, especially with poor quality, defective, or damaged discs that can affect results differently from transport to transport. Ripping to storage first using good ripping software that can assure high quality rips by detecting errors and rereading as needed.   Streaming  then tends to produce more consistent high quality results compared to reading an optical CD in real time in order to play it. 

Note that just because a CD is or looks new does not mean it is of high quality. You find when ripping CDs to disk that the speed at which the disc can be ripped varies with disc quality and disc quality cannot be determined with visual inspection alone.

Can you elaborate on what the sound differences were and to what extent? Differences in dynamics, detail, sound stage and imaging I am guessing?

Also what is the DAC?

Thanks.

Toslink often sounds worse than coax, in spite of immunity to external electrical noise or not creating ground loops.  It is because optical transmitters are slow.  Light is moving fast, of course, but transitions are slow making moment of detecting threshold voltage vulnerable to system noise.  This will produce slight time variations (jitter) resulting in added noise after D/A conversion.  It is possible that your friend's transport had faster optical transmitters (LEDs) or had less system noise (better power supply, better shielding etc).   Coax has much faster transitions (tenfold), but it has other problems like sensitivity to electrical noise, ground loops and reflections in the cable.

@kijanki 
 
Some DACs perform better with Toslink than USB/coax/etc., so a blanket statement about which input is best for a specific DAC can’t be made. 

Assuming transports are working properly, there is also often errors in reading optical discs in real time as CD players must do, especially with poor quality, defective, or damaged discs that can affect results differently from transport to transport.

What Mapman said.
I’ll add the standard being used to transfer the datastream to the DAC; the use of USB, SPDIF coax, AES, or optical. Also the quality of the cable: ie, how well a SPDIF cable faithfully transfers data without adding jitter.

And IME, the design of the transport; I went from using an NAD CDP as a transport to an ARC which was a major upgrade in SQ, both as a CD and as a transport. My quality of playback increased again since adding a PS Audio memory player.

Some DACs perform better with Toslink than USB/coax/etc., so a blanket statement about which input is best for a specific DAC can’t be made.

That's why I said "often".