I need some suggestions for music tracks to audition with my newly acquired HD800 (not HD800s) phones. Bought secondhand from a local meticulous audiophile who upgraded to HE1000SE's to keep his Focal Utopia's company when he's away These are like-new tricked out HD800's (which he preferred...

Need recommendations for best sub-$100 USB headphone amp/DAC to match best ~$100 headphones, taking into account driving capability/matching.  Prices may be for used.  Under $100 for each is target.   Headphones under consideration from the "Best ~$100 Headphone thread"   AKG K240...