[FatBox360]

Question: What accounts more for the hiss with the SRH1540 and Vali; the impedance or the efficiency?  Why don't the HE-400s hiss on the Vali?
 
Scenario: The HiFiMan and Senns plug in to the Vali and I get no hiss (noise floor?).  Plugging the Shures in and I hear a very audible hissing.  No hiss/noise with any of the cans on either of the other two amps (Asgard and Alpen).  I knew the Senns were going to be the safest bet with the Vali as far as impedance matching.  The HiFiMans sound ok; I prefer them with the Asgard.  But there's certainly no noise compared to the Shures on the Vali.  The HiFiMans even have a lower impedance, so that got me questioning if that 6.5 dB SPL/mW difference is really the difference.  Or maybe it's something to do with the mechanics of the headphones; planar vs dynamic?
 
If I'm not crazy, the Hybrid Noir cable from Forza Audioworks that I have for the SRH1540s is also a little noisier than the stock cable.
 
Thoughts?
 
Headphones

1. Shure SRH1540 (46 Ω, 99.0 dB SPL/mW)
2. Sennheiser HD-600 (300 Ω, 97.0 dB SPL/mW)
3. HiFiMan HE-400 (35 Ω, 92.5 dB SPL/mW)

​Amps

1. Schiit Vali (6.5 Ω output impedance)
2. Schiit Asgard (2.0 Ω output impedance) 
3. Fiio E17 Alpen (0.3 Ω output impedance)