Hey BC, it has been a while. Yeah with your super ears you probably need to talk to Michael & his staff 'cause just because I don't hear anything but a very black background on mine with any of my IEMS won't mean it won't sound to you like being inside a subway tunnel with a train passing.
I have a number of very sensitive IEMs that I've tried with my DACmini & I've had zero problems. In fact my unit's gain was "upped" for me to allow it to match better with my hard to drive full-size phones & still don't hear any hiss with any of my IEMs including my favorites, Westone3, 3X10s, or Monster Coppers. I no longer have any custom fit IEMs, thanks to our new puppy.
If you're interested in the mini, please talk to Micheal or one of the other CEntrance guys & I bet they can tune a CX for even your sensitive ear. I for one would be very interested in "hearing" bach from you here after you've had a chance to try a mini out. I think it was on the Nuforce HDP thread where we exchanged some posts concerning that unit's "background hiss" issue with you, though I could be wrong. I monitor lots of threads though I seldom actually post so it could have been another thread.