Hi Julian, I'm glad you are enjoying the M-DAC as much as me. I love the Audiolab and Denon combo! I have found a few other headphones that shine on the M-DAC, but not everything seems to get quite the most out of it. That's not to say that anything sounds bad, just that some thing sound better on another DAC that is about half the price. Some of this is natural presentation synergy.
Another weird story is that RHA sent me their entry level headphones to play with recently, these £30 headphones get a massive boost in quality on the M-DAC. I know it's not really a sane thing to do, but I just expected very little change. Also the biggest flaw with the RHA CA200 is that the treble can be really harsh, but toning down the treble is not a skill that I wouldn't usually associate with the M-DAC (probably the head-amps fault rather than the DAC) so it's doubly confusing.
As far as burn in goes - I don't notice much after the first day or so, I put the rest of any experience shift down to us getting used to the sound rather than it actually changing.
Speaking of amplification, I tried the
Shonyun SH-301 connected to the M-DAC yesterday and it was very impressive! Although I haven't tried it with the Denon headphones yet. So far I have tried them on the Mad Dog, DT770/32 and the GermanMAESTRO 8.35D, the latter was really stunning because it gained a nice sense of smoothness. I'm not concluding that the M-DAC's amplifier is poor, because I don't think it is, just that this combo can be very interesting. I haven't had much experience with dedicated headphone amplifiers yet so I wouldn't like to say too much more here.
Oh and speaking of Denon, have you seen that you can get the new D7100 on eBay for £449? That's one heck of a discount and it's an official distributor! Now I don't like the look of them much, but for this price I'm almost willing to take a punt.