Recently picked up a Schiit Mjolnir 3 headphone amp. This amp is an amazing performer. Provides a lot of flexibility with numerous options for configurations. It easily competes with my other headphone amp, which is a Aurouasound HEADA.
Read a few reviews, amd they all recommend for most lower impedance headphones, use low gain, single ended, and zero feedback.
With a Topping DX9 as the DAC, both the Grado Signature HP100 and the Dan Clark E3 have never sounded better.
The Mjolnir 3 is a keeper.