I just got off the phone with headphone.com. Where I got my Shures 846 from. I asked them what can I do to fix the hissing problem with both my 2011 iMac (pretty much maxed) and late 2013 15" MBP (base model). The hissing was horrible on my iMac, and he said it's because the 846 are so sensitive that its picking up noise floor from the desktop. I told them that I recently purchased Dragonfly v1.2 and while I saw a lot of improvement over both system, the hiss was still there.
He suggested since the 846 is so sensitive (9 ohms) that it really needs a DAC with output impedance lower then .5 ohms. The Dragonfly output impedance is .65 ohms, slightly shy from what the 846 needs.
He suggested since the IEM are so sensitive, I should get Etymotic ER-4P to ER-S adapter. $50 from their website.
http://www.headphone.com/products/etymotic-er-4p-to-er-4s-adapter
- Short Cable Adds 75-ohm Impedance to Audio Signal
- Perfect for Minimizing Background Line Noise
- Ideal for Using In-Ear Earphones with Any Headphone Amp or Stereo Component
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- Compatible with All Earphone Brands
So looks like this is the only option to fix the hissing from the 846. I found a cheaper adapter on ebay.
http://www.ebay.com/itm/290372905689?_trksid=p2055119.m1438.l2648&ssPageName=STRK%3AMEBIDX%3AIT
Will getting this adapter degrade the SQ because you're adding 75 ohms to the IEM. He said you will need to raise the volume up more to compensate the extra ohms.