there is a kit you can buy for a small headphone amp, a nutube in combination with a opa. it sounds nice, a mix between a very tubby millet portable hybrid amp and a solide state amp, not neutral. sweet highs and not overly smooth mids, nice wide soundstage, lows are fine and deep but not super sharp. for the price it is a fair offer, bad thing is that the nutube is verry microphonic and picks up electronic noise, was ok after put it in a case and secured it (with hot glue, the nutube don't get hot).
https://www.tindie.com/products/microwavemont/nutube-amp-kit-a-vacuum-tube-in-new-century/