Does anything - apart from bit rate and DAC-quality (and POSSIBLY cables) - affect the quality...

The power supply, pcb layout, analogue output stage...lots of things

* Accurate audio data. Aka ripped from CD using a secure ripper.
* Lossy or lossless encoded audio data.
* Build quality of the part involved sending the audio signal to the DAC.
* Playback software. Some are bit-perfect, while others are not.
* More, I am sure...

The D/A needs two things:

1) accurate data
2) accurate timing of the data

#1 is easy to achieve. Errors in digital systems are rare, both CD players and computer-driven.

It's #2 that is very difficult to achieve. The goal is to have a clock with jitter as low as the one used to make the orginal digital A/D recording in the studio.

Jitter is the variation in high-frequency timing of the digital signals.

To achieve low-jitter, one needs a very good low-jitter clock to begin-with, whether it is in the Squeezebox, the CD transport or the computer itself driving USB.

Once you have a good clock in the source device, then the challenge is to keep the jitter low all the way to the D/A converter chip. This is much more difficult than it seems. Even the digital cables or USB cables can add jitter to the mix. Most device actually add jitter as the data makes it's way to the D/A.

This is why reclockers are so interesting. This is a device that can be located close to the D/A converter and will achieve the lowest jitter possible. It can also have I2S interface to the D/A rather than S/PDIF coax. I2S is the "native" interface of the D/A chip. These things help to keep the jitter as low as possible.

There are probably some that will chime in here and try to convince you that jitter is not important, or inaudible. They have not have much experience and know little of the problem and probably have not heard a low-jitter system. They will eventually get it, given time and experience. Jitter is what puts a really high-end digital source apart from lesser sources. I've been designing digital hardware for 30 years and I am an authority on the subject. Here are two white-papers that I wrote for PFOnline:

http://www.positive-feedback.com/Issue22/nugent.htm

http://www.positive-feedback.com/Issue14/spdif.htm

Steve N.
Empirical Audio