TubeMagic D1 hiss?

Recently decided to make a foray into better quality audio with the purchase of a TubeMagic D1 DAC.  Don't have it hooked up to my speakers yet since I'm waiting on some monoprice cables but I partially got it for my headphones anyway, so that's fine.

Problem is, when I plug my headphones--Audio Technica ATH-M50--in, I get an audible hiss.  It's not that loud, but it's noticable, and annoying.  Can't really bear to listen when ever pause is covered by a hiss.  The hiss is constant so long as the DAC is powered on, regardless of volume or of source selection or if any sources are plugged in at all.  It remains hissing after it's powered off for a few seconds, gets slightly louder, then cuts out.

Thing is, I tried with my AKG K 99 on a whim and I do NOT hear any hissing.  Which is extra weird since they're lower impedance.  I've been using the ATH-M50 for listening to my iPhone, playing my PSP, and plugged into my sound card for gaming without any hiss.

So I'm not sure if my headphones have something wrong, or my new DAC has something wrong, or what.  Any ideas?

It does have a three-prong plug, the one it came with, but I spoke too soon.  Tried with my K 99 pair again and I can faintly make out the hiss (mostly at the end, when I turn it off, since like I said, it gets slightly louder then suddenly cuts, and that makes it a lot easier to notice).  The ATH-M50 just make it louder.

Triple-checked with ear buds.  They're as noisy as the Audio Technicas.

So I guess my new DAC is a bust.