The Elex will be effected by the amps output impedance more than the HD6XX (80 ohms vs 300)
As the output impedance goes up the bass will get louder (also effects damping) and this is probably not what you want with the Elex as it is already a little warm. Typically tube amps have higher output impedances than solid state amps but there are no absolutes.
I have never tried mine on a tube amp and honestly I am not to up and up on my tube amp knowledge.
Here is a detailed article on amp impedance’s effect on headphones.
https://www.innerfidelity.com/content/musings-headphone-amplifier-output-impedance
As the output impedance goes up the bass will get louder (also effects damping) and this is probably not what you want with the Elex as it is already a little warm. Typically tube amps have higher output impedances than solid state amps but there are no absolutes.
I have never tried mine on a tube amp and honestly I am not to up and up on my tube amp knowledge.
Here is a detailed article on amp impedance’s effect on headphones.
https://www.innerfidelity.com/content/musings-headphone-amplifier-output-impedance