There are many very comprehensive reviews of the ST/STX on the web, from gamers through to Stereophile.
The headphone amp design is very similar to FiiO E9, I believe, and many people here seem to rate the E9.
I, and many others consider it just about the best value computer audio solution available; not only do you have a good DAC, but you also have a good ADC if you wish to digitise an analog source or do audio testing.
I would have suggested it earlier but here it's above the $100 you mentioned at the start.
You can also go further by replacing the op amps with LM4562 all round if you feel the need.
Unreserved recommendation from me.