Don't get me wrong, the volume is enough. It's extreme clarity and lack of hissing from computer components is a welcomed change. you don't notice how much that stuff leaks into your audio until you get a clean amp/dac audio source. That being said, I believe when I read the info about these that they are rated for 30ohm to 300ohm, check me I could be wrong but the sundara's are 40ohm and I have to jack my PC volume and my AMP/DAC volume up all the way. I spents 350 on headphones, I figured a good balance would be 100$ on an amp/dac I was wrong, I might return and get the Fiio K5 instead. But yeah, if you have some of the lower end Sennheisers and just want to bump up the volume, bass treble or lower treble and bass this is probably for you but if you are spending 100$ on an amp DAC chances are you spent more than that on your headphones and they will require more power than this can supply.
I ended up biting the bullet and paying more for the k7 pro by fiio. Fosi is kinda the low end knock off brand of them. I must say it's waayyyy better. Takes up more space on my desk which I don't have a lot of anymore but yeah it's nicer. I don't have to blast the volume and I get a nice listening experience and there's extra inputs. So if you're getting the sundaras from hifiman like I did make sure you have enough for the k7 not this because this is not enough power for them. They are harder to drive then you'd expect. No offence to this company I'm sure certain headphones sound great with this I was just underwhelmed and am returning it.