Especially the Valhalla is designed for a wider range of headphones.
"While most tube OTL amps are really only designed for 300 ohm Senns and 600 ohm Beyers,
Valhalla 2 stretches to include headphones that you’d never expect to use with a tube amp.
Many people use Valhalla 2 with sensitive, lower-impedance headphones as well."
Think they would match in heaven with your HD600. Cheers mate!
"While most tube OTL amps are really only designed for 300 ohm Senns and 600 ohm Beyers,
Valhalla 2 stretches to include headphones that you’d never expect to use with a tube amp.
Many people use Valhalla 2 with sensitive, lower-impedance headphones as well."
Think they would match in heaven with your HD600. Cheers mate!