The hd518 doesn't respond very well to amplification. But the DAC however is important.
I think the modi won't be a good match, I'm reading that alot of people have issues that it's too loud. I own a FiiO e10 and that doesn't have much room either. I think you'd be better off with the FiiO e10. Put your windows settings at 24 bits audio and set the volume at 50%. Then you have enough room.
Edit: Feel free to ask more questions.