Because CeliOS praised a lot the EQ function of E17, I'm still trying to find a way to test those difference.
I have tested again, E17 vs E17+E9, and I am more confirmed that HD600 needs an E9. E17 is quite powerful, if I did not make such comparison, I would say that HD600 pairing with E17 has already a quite good result. But listening to the same songs, having E9 you can find the music is much more dynamic and the sound is brighter not because of a high volume.
Btw, I constantly got a feeling that the song was sung in a room or "in a space"...it is not an echo, but a kind of "dreamy" feeling. I don't know it is a good or bad thing. But like Sarach Brightman, orginally I love her a lot, but now it just becomes a bit too fake. Others, like Andrea Bocelli, the music does not give me this feeling.
I have tested also the EQ function. As I said, it is repugnant to have double amplifying, not matter connecting E17 and E9 through the dock or 3.5mm. It loses too much clarity.
So, I tested the EQ function with only E17 (without E9). So, my HD600 may not have enough power.
I've said in the previous post said something negative about the treble of HD600. I meant that the extremely high frequency, like violin in "song from a secret garden", if you know, there is some part which the violin turns into very very high pinch, and I had a feeling that HD600 just can't reach there.
That's why I think an EQ is needed.
I changed my mind today.
After comparing the same music (song from a secret garden), I found that treble + 2, has already given me a feeling of artificial, and it's uncomfortable for my ear.
My current conclusion is that the EQ does not help HD600, not to mention eliminating the "veil" feeling.
Now, I go back to the original setting, E9+E17(bypass)+HD600, and listen to "song from a secret garden" and "pastorale" again.
I would now say, the high pinch is the most comfortable.
I was not satisfied with the high pinch of HD600, maybe due to I have used my Etymotic ER6 to listen Secret Garden a lot. It has a bright high, but very restricted, if not none, bass.
So, I just take out my ER6 and give it a test. The high frequency is really what I am used to. There is not much soundstage for this earphone, no bass, but the very high pinch is really amazing. I still want to say my ER6 win HD600 in this tiny bit. To conclude, I re-confirm the insufficiency of HD600 in the very very high pinch. You just do have the feeling of "itchy".
Lastly, let me add a bit information about the improvement that E17 gave to ER6, though I don't know whether there is other people using ER6 in this forum.
Similarly, there is more clarity. The bass is significantly better. (But I was told that ER6 needs an amp, which I didn't have).