Simple:
D7000 ($590) + JDS Labs Objective O2 amp ($150) + Mixamp ($130) + Fiio D3 DAC ($30, IF your source has an optical out, if not the Fiio E10 for $80+ as a USB DAC) = PROFIT
Though if you're gaming ON the PC, there are cheaper alternatives to the Mixamp and give you the same benefit. Go to the PC Gaming thread (link on the first page), for the right soundcard for you.
For a one all headphone, my vote goes to the D7000, no doubt.
And there is no way in hell the O2 can power the HE-4 properly. The HE-4 is known to need over 1watt per channel. At that point, the cheapest solution would be Audio-GD stuff, which are very powerful for the price. My NFB5 puts out around 3 watts for the HE-4. The cheaper Sparrow and NFB 12.1 also have the same power rating, though cheaper internals.
The HE-4 was noticeably struggling with the E9, which (at the HE-4's 38ohm) has more power than the O2.