Inexpensive Headphone DAC

Or the Meridian Explorer 2 DAC which even does MQA decoding!
Full 24/192 and MQA for under $200.

It is likely assembled in China but designed in the UK I think.

Regardless I have one of these too and it is quite something for the price.

I have an Audioengine and it's not bad at all.  Not sure where it's made. 

This isn't as much fun as getting suggestions - but a pretty good list of made in USA audio gear: https://americanmadeaudio.com/

Schiit Modi 3 or Modi Multibit if you have an extras 100.00.
I just got the Modi 3 to use with my Eddie Current ZDT Jr.
Sounds very good, especially for 99.00.

The Topping D50 might be another choice, but it is more clinical sounding than the Modi 3, and more than twice as expensive.
IMHO, with the new AKM chip, the Modi 3 sounds better than the Sabre chipped D50.