Question about Beyerdynamic DT 990Pro

This depends on the sound card or its headphone amplifier and, of course, on personal perception of when loud is loud enough. The lower the impedance, the less juice the sound card has to use to achieve the same volume. For me, these headphones are loud enough on a smartphone at full volume. A PC usually has more juice than a smartphone. So, yes, you get enough volume out, unlike the 600 ohm model in some circumstances.
I have a professional sound card, so 600 ohms is no problem.