I think the AD2000 sounds OK directly from an iPod headphone out. I also think the RS-1 sounds OK with an iPod.
The AD2000 has a distinctive "airy" quality that probably adds the sense of air and space to unamped sources that may be particularly deficient. It also has relatively bright highs without harshness or sibilance that can add treble sparkle to the unamped source.
I think the AD2000 sounds better with an iPod than the L3000 or W2002. The L3000 and W2002 seem to be more difficult to match with an amp. The AD2000 seems to sound good with the limited number of amps I've tried it with.
To me, its more versatile than the RS-1, HD650 or DT880. The RS-1 is best for fast-paced, energetic music and is unbeatable for sheer jump up and down fun. The HD650 is excellent with classical and jazz, and it has a smooth relaxing sound that never fatigues. The DT880 has an expansive soundstage that's great with downtempo electronica, one of my favorite types of music.
However, to me, the AD2000 can do most of the above and has qualities the others lack. It has similar forwardness to the RS-1 and can be as energetic with fast music. It is more neutral and accurate-sounding, and is never harsh. The RS-1 can be fatiguing and ear-piercing sometimes, even at low volume levels, while the AD2000 is not.
Compared to the HD650, the AD2000 seems clearer. The HD650 can seem too distant and "veiled" (aargh, that word, sorry) in comparison. Switching from the HD650 to the AD2000 always results in some re-orientation. The HD650 is smooth with rolled-off, non-fatiguing treble, while the AD2000 is smooth with bright, clear, non-fatiguing treble.
The DT880 has a bigger soundstage that is better with some music. However, its treble can sometimes be harsh, and its bass can be too thin. The AD2000 loses in terms of soundstage size but I think it has better treble, midrange and bass than the DT880.
Oops, I didn't mean to be so wordy. OK, shutting up now.