I was under the impression that the E17+E9 gives better performance than the E7+E9?
To quote a friend:
"Keep the E9 and return the E7. Return it for the E17, it's much better than the E7. You could also buy a more expensive sound card, so that you don't need the E17 at all. Stick with the E9 though, it's great."
I guess the E17 does come with a better DAC chip then the E7.
and the E17 comes with a better headphone amplifier then the E7.
The E17 has more features then the E7.
The different between the E9 and E17 is the E17 headphone amplifier is rated up to 300-Ohms, the E9 up to 600-Ohms.
The E9 comes with 3 op-amps that are replaceable(?), the E17 comes with one non-replaceable op-amp.
the E9 is AC powered, the E17 uses a battery.
The E9 comes with an impedance of 10-Ohm, which is a (minor) negative for headphone under 80-Ohms.
The E17 has a much lower impedance, which is great if your using headphones that are a low 25-Ohm or even 16-Ohm (IEMs).
You would need a sound card for surround sound for movies and gaming.
With music you really have no need for a sound card.