Behringer Microamp HA400 Headphone AmpIf you're tracking more than one musician at a time, or if everyone in the group wants to listen to the playback, youre need a monitor distribution system. A distribution system allows everyone to listen in, and control the volume of his or her own headphones. The compact and flexible HA400 can power up to 4 pairs of headphones and up to 4 independent volumes, with outstanding audio quality even at maximum volume! Ultra compact headphone amplifier