I use the S.M.S.L. sApII Pro to drive 300Ohm Sennheiser HD600s and it performs really well. It delivers plenty of clean, flat power to drive my headphones with authority, just what a headphone amplifier should do in my opinion. There is not much information around but I would say the amp is designed for high impedance headphones.  I do not recommend getting this amplifier to use with low impedance headphones as there is channel imbalance at low volume. I can hear this while listening to IEMs at lowest audible volume.   The amp comes with a source...

Overview At a retail price of $119 the 9018D has a lot going for it. The high resolution Sabre ES9018 chip coupled with an asynchronous USB receiver provides a crystal-clear sound and is a significant upgrade compared to the sound card of my MacBook Pro. As an owner of the regular 9018 the most apparent upgrades of the 9018D are a higher max volume output, digital volume control and the aluminum casing. As a company previously known for DIY products HiFime has stepped up their game with this DAC. Forget about plastics and enjoy the high quality feel...

Overview The HiFime 9018 is a definite improvement compared to the sound card of my 2014 Macbook Pro. It sounds very natural and detailed. I would still recommend an amp to drive high impedance headphones like the HD650 (mentioned in the HiFime product description) and my pair of DT990 250 Ohm though… The DAC will drive them to a loud listening volume but nothing more. HiFime informed me that in order to make the DAC work with a variety of headphones they limited the output power as the Savitech USB receiver only has a 8 step volume control. The...