i think they're build quality sucks and also their quality control sucks.
just because something is heavy does not mean it was built well.
my mini jack on the front of my amp works sometimes and doesn't work most others. not a huge thing, but i have to use a 1/4" adapter for every headphone i try it in (not that you can use much other than the hd580/600..). the major problem that i have with this is, that when i first ordered the mg head dt, headroom sent it to me and the mini jack didn't work at all. i was making much less money then and it was quite an upsetting moment. they, of course, sent me a new one and i returned the old one only to find that after a few months the mini jack doesn't work again. i barely even use the mini jack, i could understand if i unplugged and replugged it multiple times a day.
if you want quality, don't buy antique sound labs. the sound is pretty good, but i'd really recommend getting something else for the price range (a little, meta42, emp, etc.)
please note i have not heard the meta42 or the emp, but from the reviews it sure sounds like an eric-built meta42 has better build quality than the mg head..