Lastly and this is a very subjective question I know, how good is the headphone out on the mdac, is anyone here using this often and how are you finding it?
Just for a bit of reference I don't currently have anything like an amazing setup compared to a lot of people on here I imagine, I do most of my music listening through an Arcam rdac/CD73 going into a Arcam A65 amp then out the headphone out on that.
I use the M-DAC headphone out only (waiting for MIMP amplifier), the headphones being AKG K-702 and AKG K-240 MKII. I also own Arcam rPAC (which I use with the K240 when being mobile).
Using the K240, there's certainly a step-up going from rPAC's own amplifier to MDAC. I've also tried various headphone amps, but I never got as good experience as with MDAC. Blind tested, I'm able to reliably identify both, even on such "lowend" headphones - either I simply learned how each DAC sounds or something else affects the quality (power, amplifier, ...).
Moving on to the K702, I can tell something more about the difference - when using an external headphone amp for the rPAC (it itself doesn't really have the power), the main advantage of MDAC was the ... "assertivity". Even on complex music, it always kept the sound "under control". If you're a fan of instruments blending together, then either don't get AKG K702 (or MDAC if you already have them).
I personally love the effect - when a recording is mastered well, one can point by hand where (in front of the stage) each of the 10 instruments is playing. This doesn't seem to happen with the rPAC.
The second killer feature of MDAC are the filters - I'd love to have them on rPAC and sometimes I really wish the MDAC was more portable. Want to enjoy a balanced music? Switch to Optimal Transient XD. Want to grasp every tiny detail? Switch to Optimal Spectrum.
On a more "spec" note, the headphone amp on the MDAC is class A, if I'm not mistaken.