What do you mean by "...it doesn't do them justice..."? I'm afraid that is simply how 880s sound :-p
Your Scarlett 2i4 interface should actually be a pretty good match for the DT880s.
I'd be surprised if you don't get enough volume out of this setup. None of the DT series headphones are particularly hard to drive despite their reputation (i.e. 600 ohm). So I would simply start by downloading the Scarlett's manual from the official site and getting the interface set up correctly.
The manual should also explain how to connect an external amp: