If you’re tracking more than one musician at once, or if everyone in a group wants to listen to a playback, you need a monitor distribution system. The compact, flexible Behringer HA400 can power up to four pairs of headphones and up to four independent volumes.
You can use this super-compact stereo headphone amplifier with practically any headphones – four of them at the same time, to be exact! Let up to 4 different people in your studio listen to the main mix while recording, and each listener can determine their own volume level using the dedicated output level controls. The Behringer HA400 contains 4 high-power stereo amplifiers that maintain the highest sonic quality even at maximum volume levels. Ultra low-noise 4580 operational amplifiers are included for outstanding audio performance – these are the same op amps found in “full-size” audio equipment – plus, a DC 12-Volt adapter is included.
Here is Review of The Behringer HA400 Headphone Amplifier