Sorry, seems I confused you. What I was trying to say is: the D2 is a rather powerful player, and a 16 Ohm phone plus ~36 Ohm (or even ~60 Ohm) impedance adapter gives you still loud enough sound and fixes roll-off, crosstalk, and hiss.
There's no need for the FiiO E5 at all in that setup, since the FiiO won't make anything "better" that an impedance adapter can't do as well, or better, for these kinds of IEMs. You can save yourself the hassle of carrying around an additional amp and having to charge it, etc.
As for the FiiO's bass boost, the D2's BBE Mach3Bass sounds quite a bit better to my ears. The bass boost might be a reason to use the FiiO with players without decent EQ (iPod, Zune, Creative, etc), but for Cowons it wouldn't be overly useful.