Indeed, that is a bit of a paradox. The EP630, V-Moda Vibe, and most others are 16 Ohm drivers with generally (IIRC) higher sensitivity than the 26 Ohm HJE900. I don't have all of those, but I know that my (very similar) CX300 hiss rather badly with most of my players, while the HJE900 definitely don't.
Now I don't have any stock sound card in any computer to try, but I know that for example newer Realtek sound cards do a kind of impedance matching/sensing with an attached headphone load, so it might be that such sound cards automatically boost the output too high with the HJE900, and use a more tame amp setting with the EP630. That's the only explanation I can think of.
Now I wonder if an inline volume control or an impedance adapter would remove the hiss from the HJE with such sound cards, or if it would make it worse.