A client asked me this question today, and I thought I might as well post my answer up as I had a moment of clarity and it seemed like a shame to share it with just one person:

When it comes to quality, dub plates don't always sound better, you get good dubs and bad dubs. Dubs often sound good because they are almost always produced professionally, but in terms of quality they are not better per se, except perhaps they are analogue and have superior "sample rate" capabilities (you can record over 20khz). If you get a CD mastered properly it should sound fine. Even 320 mp3s can sound OK too.

However, dub plates do add a certain character to the sound which is difficult to explain in technical terms, and impossible to create artificially. Also, perhaps the reason digital masters sometimes have a bad reputation among DJs is because of the "loudness war"? Perhaps its because anyone can burn a CD? Who knows?

Suffice to say any intrinsic difference in listening pleasure between the two is probably a matter of taste, rather than technical specification. I guess the question that one should ask is: "what is it about the process of cutting dubs (and especially master lacquers) that gives them a good reputation for quality as opposed to CDs?". Its a bit of a mouthful I know, but I think its more to the point.

Firstly, cutting vinyl is extremely difficult, and requires a lot of skill and patience. This is bound to increase the chances of vinyl coming out sounding better, as the person at the controls is more likely to know what they are doing.

Secondly, CDs that are cut too loud can sometimes slip underneath the radar until the listening fatigue sets in. Vinyl does not sound good when its cut too hot, not even for a single beat.

Thirdly, vinyl sticks around for a long time. There is a wealth of reference material available, and to a large degree our current taste has been formed by years of listening to vinyl. Digital audio is currently riding on the peak of a loudness war and its not yet clear when the wave will break and things will reach a sensible level.

Of course, there is more to it than this, but the point is its futile arguing the case for vinyl on the basis of quality alone. If you prefer using vinyl its probably for personal reasons more than anything, but if you are wondering why it often sounds better quality, its almost certainly due to the mastering rather than the format itself.