beyerdynamic DT 990 PRO Studio Headphones Ninja UK | Ubuy
Zoom ZHA-4 4-Channel, Battery-Powered Headphone Amplifier with Volume & Mute Functions, for Podcasting, Music, Production, and More. : Amazon.co.uk: Musical Instruments & DJ
HM4 Four Channel Personal Monitor Headphone Amp | Miktek Audio
my grandfather gave me those 4 channel headphones. : r/headphones