I really like the Oppo HA-2SE Portable Headphone Amplifier and DAC. High-resolution USB DAC with the ESS Sabre32 Reference ES9028-Q2M DAC chip
I have had a number of other headphone amps, but this one tops them. I use it with open and closed backs (Sennheiser, Oppo, AKG, etc.) and with IEMs (Shure, Etymotic, etc.). Right up there with my tube EarMax but super portable.
Cheers,
Eric