Another interesting option for you -- Schiit has a solid state 6SN7 tube available to replace vacuum tubes. You could get two of those for the headphone section and then you'd never have to worry running down the tube life while they are sitting there doing nothing. And, if you decide to use headphones, that circuit is immediately available.
If you go to Schiit's web site, and look in "Accessories" under their product lineup, you'll see them listed as LISST tubes. I bought one for my Lyr 3 headphone/preamp and was quite impressed with how good it sounds. I very slightly prefer the glass 6SN7 but was surprised as to how close the LISST was to the glass tube.
Finally, they do have a disclaimer about using these in amps made by other companies, so be forewarned. However, I'd suspect that is mainly CYA wording from their lawyers. But, still something to consider.