This evolution of Morley's classic volume pedals is passive, which means it doesn't require any power: just plug it in and get swelling.
The M2VO has a smooth taper, which makes for precise transitions, whether you stick it post-dirt for volume adjustments or pre-dirt to control your overdrive levels.
It does rob a little treble compared with plugging straight into your amp, but no more than any other passive volume pedal, while its weighty 1.6kg enclosure makes it a seriously substantial bit of kit, too.