Amazon.com: Neewer Super Compact 4-Channel Stereo Headphone Amplifier with DC 12V Power Adapter for Sound Reinforcement, Studio, Stage, Choir, Personal Recording, Features Ultra Low Noise (Original Version) : Musical Instruments
Dirty cheap behringer ha400, a few thoughts : r/headphones
Behringer HA400, 4 Channel Headphone Amp, Also Boosts Sound to Portable Devices | eBay