Sorry for the trouble you have had with the C5 unit. If the replacement is still not up to standard, please contact me, I'll see what I can do.
The C5 is slow-moving at the moment, we think it is still a very capable portable amplifier, but the market tends to sought after the newer and more expensive products.
I do suggest C5 users upgrade their 3.5mm cable, the short cable that comes with C5 is barely functional. At the price we charge, we only provide a cable to test the C5 functional when out of the box. Its not necessary to spend big money on that, a $10-$20 cable should work fine already.
The high gain of C5 sounds warm with good body, some users even described the C5 high gain as tube sound. But the high gain will output 800mW into 32ohm, that's around 30 times more powerful then the headphone output of an iPhone 6. This is certainly a lot of power for sensitive headphones such as M100, so please be very care when you switch the C5 to high gain.