You will find a lot of debate on this topic in the audiophile world - which is better an (e.g.) €5000 krell/meridian/wadia or a cheapo dvd player connected to a higher quality dac then into your amp.
But your question needs to be qualified. A standard cd player has two main functions - first read the cd, extract the bits and then convert them into analogue signals for amplification. This last bit is a key differentiator on many cd players - the dac (digital audio converter) and the analogue output stage.
Jitter is the timing error between the sequencing of the digital samples on recording and the decoding for playback. More expensive cd players are marketed as having better dacs which are better at handling jitter. The dac itself will be responsible for turning the bits into music - so itself is responsible for the "flavour" of the sound coming off your cd. The extraction part of the machine is usually based on a standard mechanism, but again there can be huge differences - key thing to consider here is timing - can the machine resolve all of the information including error processing quickly enough. e.g. Meridian solve this problem by buffering cd audio data in memory before the dac.
A digital amp is one which takes the cd data directly and decodes it itself. There are not many good quality digital amps on the market ie. one that directly reads an digital audio signal, converts to analogue and then sends it on to power amp or speakers. Offhand, I can only think of one - TacT.
If you're talking about an av amp, then you may have problems as a) they're usually no good at all for stereo audio and b) their digital input is for ac3(dolby digital)/dts signals - not red book cd audio.
In the hierachy of digital outputs, optical is usually considered inferior to co-axial (can't explain why, but its probably due to cheap optical circuits not being up to the job of timing)
But my advice to you would be to let your ears determine the difference. In my limited experience, cd players do make small differences here and there, but while cheapos reproduce the basic notes, better quality units make you "feel" the music.