Here's what Cornerstone say about the FlameXS:
The FlameXS has been designed out of a need for a stand-alone totally clean boost; its aim is to push up the signal volume without altering the original tone or adding any unwanted distortion.
Super simple to use, it has just one Gain knob. With the Gain knob at 0% there’s unity of volume, but the FlameXS is already warming up the sound. This is the reason why many guitarists like to use the FlameXS with the Gain knob at minimum. The more the Gain knob is turned clockwise the more the volume increases, up to 20dB of clean boost, without ever becoming saturated or overdriven.
Thanks to its micro size, the FlameXS can fit onto every pedalboard. The FlameXS requires a standard 9V DC Negative power supply only.