The PA-1 from Whirlwind is a portable headphone monitor that's great for use with headphone distribution systems, for boosting weak headphone outputs, and more. It will accept balanced or unbalanced line-level stereo signals through XLR or TRS inputs, while 3.5mm and 1/4" outputs let you use any pair of headphones. It has volume and balance controls, as well as a stereo/mono switch, and can be powered by two 9V batteries or the included external power supply.