I'm sure the aune t1 dac can be improved upon but it's about bang for your buck. We all (mostly) have to live within a budget.
Given the Aune T1 performance and the ability to roll the valves to change the sound signature to match you headphones, I doubt there is anything more flexible this side of 300.
The chatter on the forum also suggests the aune greatly benefits from a general upgrading of the capacitors. So there is still further room for improvement.
As to your other question, as I said, the bossfet is so neutral and transparent that I hear the tube signature perhaps even better than before. It's not a negative to have good amplification connected to the difficult (comparatively) load of the headphones, controlling the speaker cone. The aune does an excellent job in this setup, driving the easier standard phone load. I will have to upgrade the caps in the future though.
Many thanks on the forum for all the pics on the best way to upgrade the caps and which ones to switch out. A picture says a thousand words.
As to the Audio-GD NFB-10ES3 I've not heard it. I'm happy with what I have at the moment and I have stopped looking at other equipment reviews (that's a lie I did read the review here on the new HE-560, damn I've started drooling again).
If your patient someone will eventually do a comparison.