I have just acquired a pair of ws99, and am not sure what amp should I choose. The budget is 50-150 usd.
Also, suggestions for gears that work with mac pro is also welcomed!
Is there a reason you think you need an amplifier? That series of headphones are built to not need external amplification. They're very efficient. 102 db/mw and 37ohms. No amplifier needed here.
Now, if you want a nice portable DAC/AMP that will be better quality in general to use with that Mac Pro, and on the side, function as a portable amplifier for any headphone (again not that you need this with the WS99), then the Fiio E07k or Fiio E17 comes to mind.