HeadbangerYou're likely using low impedance headphones with an OTL amp.
However... one way to get the tubes to stop buzzing is to burn them in by leaving the amp on with ONLY one tube at a time in the socket. After burning in for about 48 hours the buzzing usually goes away.
It's a thing with tubes in general, not just this amp. Gas gets trapped in the tube and needs to burn off.