I came on here to post Centrance, as well. They are really good.
Another good option would be to get a Schiit stack. They just introduced a $99 DAC (Modi) and a $99 headphone amp (Magni) - both made in the USA. The Modi is an asynchronous USB DAC. With a $500 budget, you could upgrade either the DAC (Bifrost USB $449) or the HPA (Asgard $249 Class A or Valhalla $349 single ended tube) and do really well.
Another good option would be to get a Schiit stack. They just introduced a $99 DAC (Modi) and a $99 headphone amp (Magni) - both made in the USA. The Modi is an asynchronous USB DAC. With a $500 budget, you could upgrade either the DAC (Bifrost USB $449) or the HPA (Asgard $249 Class A or Valhalla $349 single ended tube) and do really well.