Combo DAC/AMP for Headphones

I currently have a HiFIman EF400 DAC/AMP for my HiFiman HE1000 headphones.  I would like to consider a upgrade.   What would be a step up under 1500.  I'm open minded but kind of like R2R Dacs.     or.......

Do I keep the EF400 as a DAC only and spend the money on a amp?

Thanks in Advance