Bottom line it can happily drive headphones up to 600ohms so it will have absolutely no issues in handling your beyer dynamic 990 pro's. Also with the latest AE5 and AE7 to come with top end DACS, they are still relevant especially to those that prefer in PC cards and have good options for external DAC's as well. Yes you can by great external DAC's like the Schiit, jotunheim and the super cheap yet good FX-Audio DAC to name just a few and there are so, so many to choose for every price point..