I have now ordered the Sound BlasterX G6 and tested the headphones with it and the Scarlett 4i4 Gen 3. At this point, I can say that with the G6, a plus in volume can definitely be achieved, but the volume I can get out with the Scarlett 4i4 Gen 3 would have been completely sufficient for me personally. As a comparison for potential buyers: The DT 990 (600 Ohm) are about the same loudness with the following settings: Scarlett 4i4 Gen 3: Gain at approx. 75-80%. Sound BlasterX G6: Gain at approx. 40-45%. Hope I could help someone with this! :)

With this sound card, I would go for the 250Ohm version if I wanted it to be loud.

I use the DT990 600 Ohm on the iPad (power sufficient for me) or on the iPod (power just sufficient). These headphones are actually intended for a low impedance amplifier output. I don't know the Focusrite Scarlet.

You have to consider that you need a sound card that has enough power for the 600 ohms if you want to experience the full sound. I use it with the Creative G6 and can only recommend this combo.