I can't speak on the other two DAC options but I have an iMac (27"/3.4) and the Audioengine A5+ with the D1 DAC in my home office.  For the price I'm happy.  It's not the most transparent system there seems to be a roll-off at the top end but there definitely is some bass presence (not super accurate) but it makes the iMac infinitely more tolerable. My system of comparison is in the other room and is a pair of Revel Studios driven by Mark Levinson amplification and Nordost Valhalla cabling all around (speaker, power, interconnects).  In my other office I have a pair of Dynaudios driven by a Rotel receiver that I still prefer over the Audioengine but the second office system is over $2k excluding the price of a Sony SACD player.  The Audioengine system is by far the weakest of the three setups in terms of transparency and dynamics but you're also talking about a very low cost computer system.  I think the A5+ is the weaker offering compared to the DAC.  The DAC actually sounds pretty good when I'm running my headphones (studio monitors from Sony) through it.  You just can't expect too much from a powered bookshelf speaker at the A5+ price level. 

Sorry for the ramble.  In a nutshell, I like the Audioengine system - just be realistic about what to expect from an entry level system like this.