I am surprised to see that this SJ55 sounds neutral to some, maybe if you compare them to the AKG K18DJ it will sound neutral. But, if you are going to compare this headphone to a vast variety, this is far from neutral. This headphone rather have a "U" shaped frequency like Pioneer's DJ headphones. For a portable these are heavy on bass (cancels vibration through the aluminum sealing), recessed mids, and sparkly highs. Quite evident in showing sibilance in some tunes.
Technically, a neutral sounding portable is so hard to achieve, the hd25s maybe the closest one, but it is still colored. a dark sounding portable is easier to achieve i.e. hd25spii, aiaia tma-1, pro500mk2, px100.
And, with amps? You really don't need to amp this type of headphone, it is well-designed for portable players. If you want to take advantage of portables amps, atleast a midrange headphone will do. It will add detail, robustness, overall tighter low-ends. For an entry-level headphone, this is one of the better choices for a wide variety of music.