There's really no difference listening between digital and analog with headphones. A source is a source and the amp and headphones (if they're decent) will deliver whatever comes from the source.
What's your budget?
Do you currently have speakers? What do you like most about them? That would help in terms of pointing you towards a specific sound signature, do you like things really neutral, a little warm, or do you prefer more "analytical"? Are you a basshead, a midrange freak?
Other things to consider are comfort and build, open or closed back (will you be listening around other people?) and sensitivity matching the headphones to the amp.
Why the preference for tubes? If you want quiet, in general solid state will be more quiet, but there are some quiet tube headphone amps out there.
What's your budget?
Do you currently have speakers? What do you like most about them? That would help in terms of pointing you towards a specific sound signature, do you like things really neutral, a little warm, or do you prefer more "analytical"? Are you a basshead, a midrange freak?
Other things to consider are comfort and build, open or closed back (will you be listening around other people?) and sensitivity matching the headphones to the amp.
Why the preference for tubes? If you want quiet, in general solid state will be more quiet, but there are some quiet tube headphone amps out there.