[DaremoS]

Sound quality have been frequently confused with equalization: deep bass, vibrant midrange, crystal clear treble, etc. are terms related with proper equalization.
Sound quality is the ability to reproduce original source without distortion, without spurious noise and at the proper level. Clicking, stuttering and other issues like that have to be also with sound quality.
Nowadays most of the player manufactured have such capabilities, it has to be a really poor manufacture or electrical failures what could make a player to sound really bad.
If everything goes fine the rest are the capabilities of the player to manipulate accordingly the original sound, and here comes additional chips (or software) that can handle sound in the right way to make pleasant the enjoyment of the player. You can have here the Digital Audio Coded (Wolfson for instance) or the software (PlayFX, SRS, BBE, etc.).
Here comes too the appropriate resources of the firmware as are all the sound managers frequently added in the software (firmware) being the Equalisator the most known.
It's important her to take in account the source: a good source will bring better sound if it's good. It won't sound the same and MP3 @ 64kbps than other @ 320kbps. Here are also the format it's not the same an MP3 (lossy) than a FLAC (lossless) format.


And finally... THE PHONES... the most critical part in what have to be with the enjoyment of the sound. All what is explained in previous paragraph worth nothing if the player doesn't have a good pair of phones.
Even the cheaper player can produce a delightful sound if it has a pair of good phones.
Even the most expensive player can sound like shit if no proper phones are used.

There is no such thing as good sound players... there are limited player, faulty one, bad firmware, badly manufactured, ugly designed, etc, etc, etc. but with actual technology any decent player can bring a decent sound.

The rest is merchandising, myth... or simply placebo.