The PreSonus HP4 Discrete 4-Channel Headphone Amp features 4 headphone outputs on the front panel, each with its own volume, and 150mW of amplification per channel extremely loud output with a low -98dB noise floor. It also allows control over control room monitors with a separate volume knob that can be muted using the Monitor Mute button. 1/3 rackspace. Accepts balanced/unbalanced inputs and outputs (1/4" TRS). Input can either be set to stereo or mono (the mono selector button on the front channel simply copies both inputs to both sides). Powered by an external wall-mounted supply.The HP4 represents a tremendous value as echoed by this email received by PreSonus:"To my amazement, your HP4 sounded as good as both of my hi-fi stereo heaphone amps. Very very clean and loud. I can't believe you put 150 mw per channel, @52 ohms, to four headphone outputs and offer such a quality unit for very little over $100." (M. Burke via email)