The E9 is a desktop-only amplifier that can output a lot of power - more than what you need for the HD598, although it could be useful if you upgrade later to the HD600 or HD650 - but has 10Ω output impedance that makes the bass of the HD598 slightly boomier.
The E10 is a USB DAC+amplifier. It offers decent quality, but is less powerful than the E9.
Finally, the E17 is a combination of the E7 and E10 with some improvements over both. The amplifier part of it is similar to the E10, maybe with slightly more power. Unlike the E9 and E10, it can be used as a portable amplifier. It also has bass and treble controls.
Edited by stv014 - 1/29/12 at 11:35am