I'm huge fan of strong distortion effects. Through my adventures with guitars I've tested many metal effects.
Metal Muff have wide range of sound settings. Mostly its Lows and Mids are very visible in sound. Highs gives lots of sharpness, balanced to rest of frequencies.
Distortion sound is very clear, and deep. You feel its power.
Boost gives a huge kick in to sound sharpness, power, and volume. Real power, that can be incressed or decreesed as you like.
First feeling is like this sound is too muted or something... nothing more wrong. Its just different. This box need lots of patience and time for test many different settings.
Construction is very solid, knobs are ok, switches are solid and will stand even hardest boots.
One minus for this effect: Its dominating over other pedals on your pedal board. A bit confusing to plug it in " Your " Propper order with other effects, to not loose an edge of sound.