A pedal "two in one" incredible. Distortion type old tube amp and a boost of +20 db. In fact it is a preamp. I already tried (in this form) but they had a tendency to distort the original sound. Here is the foot.
An input jack and two outputs are available. A CLEAN OUT output (for use only boost) and OUTPUT (for use distortion and boost)

UTILIZATION

Regarding the settings. We can no longer single!
There are three knobs LEVEL WAVE GAIN to equalize the sound of the distortion. BOOST and a knob for adjusting the desired volume increase.
Even without a manual pedal is affordable. The manual provides tips quite interesting as some typical settings for blues, rock, hard rock and heavy rock and some tips on using your amplifier.

SOUND QUALITY

Attention is the key to pedal hard rock n roll. Distortion approaching that of a light amplifier (I compared with my JCM 800 LeadSerie), with settings that let you move from one rock to blues disto disto heavy rock. Playing on the guitar pickups and pedal settings we have everything we want in the field of rock. For metalheads, it's not worth it. Sad, but good!
But where it gets crazy is when you use the BOOST (pedal has two switches, a distortion and a boost). To perform a solo without the rest of the group can cover you, it's ideal. Up to 20db more. HUGE. I use the volume pot on January 1 quarter of its potential and is already almost too much. Longer need to savagely attack the strings, no need for compressor distorts the sound. The true sound of the amp louder (much louder) simply.

In addition to the bypass is a real treat. The clean sound is really clear, no noise. To this we must add that the power lies in the pedal so no transformer (and breath).

I use this pedal with a Jackson Dinky AR and a JCM 800. A real treat? Bluesy rock sound to its wholesale Heavy Rock, nothing escapes me. A true happiness.
NOTES: pedal shows its limits when you go to with the big rock in open RE #. Here I would advise the Tech21 XXL (see review)

OVERALL OPINION

After 15 days of use I wonder how I managed before Scratch'N. Everything is good in the carl martin I'd try to say. If this application is exclusively reserved for rock hard rock. The same pedal METAL version would be a killer.
Before this I tried a Pre-1/Booster Nobels, which had comparable opportunities except it had no distortion and it distorted a little sound. IT was just a boost (very effective incidentally).
It is true that the price is high but well I really think it's worth. When we bought it we understand that price, which is quite low actually.