DAC/preamp combo together with standalone preamp

I am rather confused by a lot of the new digital gadgets. I have been considering adding a DAC/preamplifier combination to my system. Am looking at various gear. Many of the DACS have a preamp in built and many of the reviews I have read (Debussy, Meridian 818 for example) have them used together with a pre-existing standalone preamplifier. What are the pros and cons of this.
Many thanks
in anticipation
Setting aside performance...which you should judge for yourself, the pros i supposed are fewer cables (saves money), potentially lower cost (1 unit not 2). The cons are that in many reviews the analog preamp part of the unit is not as good as separate preamp...but the digital bit can be phenomenal. So if you have a turntable, or other analogue inputs, in some cases, the reviews/comments suggest a separate preamp will suit better in those cases. For example, if you are looking at Meridian 808.3i, some might say go with ARC Ref 5SE for analogue inputs as preamp...but the digital DAC direct to your amp will sound great. Two cents.
Look at Linn's new DSM range which are basically a wonderful digital streamer, dac and preamp in one case.
They also have very good analog inputs on them and even include a very decent phono stage.

Cutting edge technology coupled with Linn's legendary manufacturing prowess.

The new Audio Research DSi Dac and preamp combo is also superb and should be heard by anyone looking at this type of preamp/dac combo.