are there really any differences at all?? i cant seem to find any

The 90 comes with the iPod cable, a two year warranty, and high volt pre-out (4.0v)

The 900 doesn't come with the iPod cable, has a one year warranty, and has the low volt pre-out (2.2v). Other than that they're the same.

wat does the volt output affect the spl, sq or something like that?