In a word (or two): rock solid. With every headphone I've ever attached to the M8 I've always felt the cans were being driven with great authority and control. There is always plenty of gain for those difficult to drive cans, and at the other end of the spectrum, the noise is very low and never apparent even with the most sensitive in-ear monitors. I use the M8 with my Astell&Kern AK120 with a Toslink optical digital connection and the sound just reeks of competence. Without referring to other amps directly in comparison, the fundamental impression I get is simply a wire with gain. Music is clear, articulate, dynamic, and superbly controlled. I'm of the opinion a headphone amp is the least important thing in the chain between the source and your ears, and the best thing it can do is amplify the signal without flaw and get out of the way otherwise. The M8 does that as well, or better, with virtually any headphone than any portable amp I've heard.
I'm very much interested in the CEntrance HiFi M8 portable amp. However I have so far not received any clarification about its functionality when used with an iPhone 5S. From the few user comments I have read, it seems that using the Lightning connector output of the iPhone 5S, the HiFi M8 can only decode 16-bit/48kHz. But I cannot get any official confirmation of this. I asked CEntrance and was advised that with the iPad you can use the CCK (camera connection kit) and use the Android version of the HiFi M8 to get full resolution. This doesn't tell me for sure if its the same for the iPhone 5S. Even if it is, then I'm not really interested as this adds too much clutter.
For a portable solution, I'd happily add one (not too bulky) product to my phone, but no more than that. The only other alternative I have seen is Sony's PHA-2. I need to look into this a bit more before deciding.
