FiiO K7 Full Balanced HiFi DAC Headphone Amplifier AK4493S*2, XMOS XU208 PCM384kHz DSD256,USB/Optical/Coaxial/RCA Inputs, 6.35mm/4.4mm Output

Being a balanced amplifier, it provides a lot of power and can easily drive any headphones and/or IEMs you throw at it. I've been driving my HD660S2 pretty easily with this. The volume knob is non-linear. I haven't studied it too closely, but I'd say it's logarithmic. That means you'll get very low volume upto about 11'o'clock position. After that, the volume grows much faster. Overall, loudness is not a problem at all. I keep the volume at a little west of 12 o clock. For some reason, Amazon India is showing an extremely inflated price of 44k (against an MRP of 59k). I bought this new with warranty from headphonezone.in for about 19k (against MRP of 22k). Not sure why or how the MRP itself is blown up on Amazon. For that price one can get a Schiit Magni Modi stack, which is a higher class of DAC and AMP. Bottom line, if you buy this (check prices on different sites, first), you'd likely never need another DAC/AMP. It's built like a tank and has plenty of power on tap for anything you throw at it.