Well, I had a response with links and comments that disappeared into thin air. So, this will be shorter.
Are you willing to consider used or just new? Are you set on dac/amp combo or would you consider separates?
In regards to headphones, I second the Sennheiser HD6 family. I would also look at ZMF, specifically the Atrium Open. They also have a new planar model being released that is supposed to combine the warmth/lushness of the Atrium and the detail of the Caldera. It's called the Tessidera. You might look at the Focal Clear OG as well.
When it comes to amp/dac combos made in the USA there's not a lot of options. I think Schiit, which yyzsantabarbara mentioned and you own is a good option. I nearly purchased the TEAC UD-701N which is a precursor to the UD 507 you're considering. Here are a few others to consider:
- Black Ice Audio Aries
- Violectric DHA V226
- Mytek Liberty DAC II
- ifi Pro iDSD Signature Made in UK, but 2 offices in US
- Rupert Neve Precision DAC

