This is only my second foray into IEMs, so take this with a grain of Himalayan salt...
I'm using this with a FIIO E12A, set to low gain, volume at 12 o'clock coming out of the line outputs of my Clarett 4pre USB interface. There is an electronic drum kit and guitar processing rig feeding the Clarett, and both sound excellent. Compared to my Beyer DT880s fed from a Topping A30, there is slightly more impact and slam through the EDC3 setup. I believe this may actually be slightly more accurate than the Beyers to be honest, because on my monitor playback system using Focal Solo 6's, there is also slightly more bass than the Beyers.
Music fared equally well, with a very nice tonal balance, and good impact. It offers some level of analytical insight, while also ensuring that if there's some groove in your tunes, you'll feel it.
However, I tried connecting these directly to the Clarett, as well as the outputs on the drum rig, and until he volume got high enough, there was a buzzing or humming sound. I believe you really need to use a low output impedance source on these if you want to get the best out of them. My Clarett uses a 10 Ohm output imp., which apparently causes the noise. Through the E12A, there is a pitch black background, regardless of volume setting, and I still have plenty of headroom as well. They make an outstanding pair, especially for a truly low cost, but top notch monitoring setup.
The fit of the basic plugs was good right out of the box. They broke in after about 3 days and really sealed my ears well. So, isolation was perfect.
If I could ask for anything, it would be a longer cord. Perhaps another meter.
Worth every penny....so far. Hopefully they will also prove to be reasonably durable.