[Crimac95]

FiiO M9 won't hiss with DT1990 PRO.

Keep in mind DT1990 will typically expose recording hiss much more than HD6XX.
Headphones with very extended and slightly raised treble are much more prone to expose recording hiss.

Make sure to test with recordings that you're a 100% certain they don't have hiss. And of course, skip live recordings for this test.
Once you find a proper (hiss-free) recording to test try the following:

1- FiiO M9 ---> DT1990 PRO
2- FiiO M9 (line out mode) ---> FiiO A5 (Low Gain) ---> DT1990 PRO

Let me know if you still hear any hiss in test 1 or 2.

Regards,
Me x3

Actually I tried to play with the 10 band eq of the m9 and the hiss almost disappeared taking all the way down the 9th band (2k - 4k Hz) but of course that influenced the whole song in those frequencies

 

[Me x3]

That's not very useful because if hiss is present in the recording itself instead of the setup, you won't find out.
You should hear no hiss at all, unless it's on the recording. If there's hiss in the recording, you'll surely hear it with a resolving setup like FiiO M9+DT1990PRO