Very odd, indeed...taking into account that M-DAC is a great-sounding DAC with discrete Class A headamp in one nice package at very nice price.
The M-DAC alone is a bit underpowered for the HE-500, though I didn't try driving them using the XLR outputs. The built-in amp is rather good, slightly below the Lehmann Black Cube Linear in performance. If you power the M-DAC with a high powered headphone amp it should let the HE-500 shine a bit more. I found the M-DAC a bit too aggressive sounding and lacking in bass, but that was more than a year ago when I sold my M-DAC so my memory might be out of tune.
if MDAC, Wadia 121, Teac UD-501, YULONG D200(DA8), mytek stereo192 and Audio-GD NFB1 are the same price, which one(or two) would you choose for 80% USB 44/16 and 20% new experience in hi res audio?
Only the USB sound quality have matters. I love old(jazz rock) LP and CD records before loudness.
Other equipment that I use at the moment Beta22, HE500,HD600, AKG701
I like M-DAC digital filters, use 3 of them quite frequently to slightly alter sonic signature, when I wish. Kind of "3 DACs in one".
It also handles Hi-Res up to 24/96 via USB flawlessly.
BTW, regarding Hi-Res Audio bandwagon - to my ears it's mostly marketing bluff.
I do hear differences (using ABX plugin in foobar2k) between different masters of same 16/44 album (MasterSound or MFSL releases vs 'regular' ones, for example).
Although there is no audible difference to my ears to Hi-Res vs 16/44, there are cases, when better quality master can be found only in Hi-Res format nowadays - in that case, don't bother above 24/96, it's just a waste of HDD space.