FiiO E9 + HD518s: What gain setting should I use?

Use the lowest setting that gives you adequate volume. All gain does is increase how much voltage the amp will swing. You might get better distortion and noise at low gain.

Also aim to have the volume knob between 9 and 12, as that is the E9's sweet spot. For me, it's whatever keeps me between 10 and 11 o clock.

If you are using a PC, the E7 acts as an external soundcard, which will be better than most stock onboard soundcards. If you have a really good internal soundcard, you won't gain much if any benefit from the E7.

I have some Beats Audio soundcard on my HP PC, and the E7 is noticeably better than it, as it feeds a clean, distortion free signal for the E9 to amp. Though for that purpose, the E10 is CHEAPER and better than the E7. The E7 is more expensive because it's a DAC as well as an portable amp (which you won't use if you have the E9).
Edited by Mad Lust Envy - 1/3/12 at 10:34am

If you already have the E9 and don't need the portability or amp of the E7, you'd get a better value if you got a different DAC like the Behringer UCA202. On the other hand, the E7 would be plenty powerful enough alone for the HD518, and if those headphones are anything like it's older brothers HD558 and 598, you'd get slightly more neutral bass too.

Hijacking the thread to ask a quick question. If you're using an amp with your PC, do you want to have the desktop volume setting to 100? Or should you adjust it so that your amp volume is between 9-12 o'clock? I'm using HD-600s with the NFB-12 (dac/amp) on low gain, desktop volume at 100, and I am usually sitting at 9 to 10 o'clock on the amp volume. I just thought I'd need to be using high gain or more volume from my amp to drive 300 ohms. Is my setup normal?

That sounds fine, as long as you aren't getting noticeable channel balance issues where one side is louder than the other. Contrary to popular belief, the HD650 isn't all that hard to drive. It has a fairly high sensitivity, compared to say AKG headphones or most full-sized Beyers (which at 250 ohms are just as hard to drive).

 

What would cause channel balance issues? I do feel like one channel is louder, but I just thought that my hearing was slightly better in one ear.

Some amps have channel balance issues at low volume settings, if the volume control doesn't lower each channel at exactly the same rate. Try to turn your computer volume down and amp up to about 12 o'clock, if the balance is fixed then it's a problem with the amp. If the balance stays the same, it's your ears (or headphones, or your music files, etc.)