Either gain setting will be fine for the D2000's. If you plan on using other full-sized cans (which may have higher impedance/lower sensitivity ratings) with the Mini^3, a gain of 5 will do you more justice. If you plan on using IEM's, I'd say you'd be much better off with a gain of two.
Both gain settings should have relatively similar noise floors, however, depending on your source, the ambient static/noise from that will get amplified along with your music. Make sure you're using a worthy source for your new Mini^3 and D2000's, otherwise your money will have been spent in vain!
