This is a straight up, to the point, amazing preamp/overdrive/boost. For years, people told CGC that the original Icarus, paired with various amps was a match made in heaven. They didn’t lie.
There's up to +12db of clean boost with no additional gain just from the second (volume) stage, but once you add that gain and start to clip the hybrid Russian germanium and silicon diode circuit, you won't want to come back. The pedal is Based on the acclaimed preamp from the Parabola and Météore, but the Icarus expands on it with the multiple gain stages, gently tapered highs control, and buffered/true bypass. All running elegantly at 9VDC with no noise-inducing charge pumps or other goofy frills.
To dive into the controls.... Three knobs and the truth! This preamp is useful and plenty loud, so don't be surprised to run it lower than you might with other light drives or boosts. A little goes a long way. GAIN is how much boost is applied to your input signal: All the way off is just the signal from your instrument, turning up will clip the diodes nicely. HIGHS is the amount of frequency content. It interacts with the OUTPUT knob, which controls the output volume. With GAIN all the way off, OUTPUT can still give you up to +12db of boost, so with those other controls up, you may want to keep it down. Horses for courses, they say.

The slide switch on the inside will convert this pedal between buffered and true bypass. The buffer can help maintain signal integrity for longer signal runs.