https://www.amazon.com/Magni-2-Uber-Headphone-Amplifier/dp/B00SCBX6FA
This is probably the best headphone amp for the money right now. They're clearing out old models, and the supply and price will not last.
I highly recommend planar headphones. Big fan of Fostex t50rp, and monoprice monolith m560. Both will be nearly impossible to drive with portable devices unless they're specialized devices.
https://www.amazon.com/Professional...8&qid=1508965901&sr=1-1&keywords=fostex+t50rp
Another great deal, you're basically getting the Fiio for free. But that Fiio is probably not powerful enough for the headphones they're being bundled with.
Just realized you're looking for tube gear. Planars are a great match for tube since they like high current. I have a hybrid tube amp that I do enjoy, the Little dot i+.