Hey, I already own a Scarlett 2i4 from Focusrite and I've been wanting to switch out my old half broken hd598s for some dt 990 pros. Can I hook up an ext. Headphone Amp to it and if so how would I go about doing so?
Do you mean the output impedance of the headphone amp? That's not the relevant spec.
You need to know that the amp has enough power with 250 cans to drive them. The spec for those is a max of 100mw at 250 ohms. You can use less but they won't play as loud. If you use 50mw you would lose 3db of max volume.