Since two weeks I am the proud owner of the M-DAC. I based my decision for a large part on the reactions here so thanks a lot for your contributions and information.
I'm very happy with the purchase. The sound is very detailed to my ears but at the same time very smooth (after running it in for 2 weeks). So far I have only used the USB input. Setting up foobar (WSAPI) was a breeze. As a result my first "bitperfect" test was successful right away. I use a Supra USB 2.0 cable for connection to my PC. I use Sennheiser HD-600 headpones.
On the day of purchase I brought my good old Sennheiser HD-600 to the hi-fi store for an audition and did a comparison against their Musical Fidelity M1 DAC. Some here have described the sound of the M-DAC as a little forward but to my ears this is not the case. I find the sound much more coherent and not harsh compared to the M1 DAC. The sound signature resembles my Rega Apollo. The M1 DAC sounded as if the loudness button was switched on. The bass was more defined but at the expense of a in my view harsh treble.
For some music I find the M-DAC's headphone amp missing bit of kick. Using the equalizer solves this but that kind of defeats the purpose. And with some music files the volume just doesn't get high enough. Therefore I'm looking at the Lehmann Rheinlander amp. Does anybody here perhaps have this combination?
I also have the relatively cheap Bravo audio Ocean tube headphone. This one doesn't gel well together with the M-DAC unless I active the internal pre-amp in the M-DAC and lower the volume significantly else I hear clear distortion. The sound with the tube amp has a bit more authority in the lower region. While some details are lost using this amp (I still need to replace the stock tubes) it surprises me that the sound signature is not that different from using the internal headphone amp.