I bought mine three weeks ago and it's a good entry level dac/amp. I have easy to drive headphones so I can't talk about raw power.
With my SRH440 (and HM5 pads) the soundstage is noticeable large and the bass is more punchy. The cans work just fine without an amp but there is a improvement if you are used to the sound as much as I do.
I have also a pair of VE Monk+ and I will test these in the next weeks too and edit this post. I listened a couple of hours and I'm not able to say anything.
There is definitively an audible noise but I guess that is because my cans are way to easy to drive to this amp (at 9 o'clock is loud, I have to turn the knob to 8 o'clock or even less). A channel imbalance at very low volume is also present in my setup but like I said, with more power hungry cans this should not be an issue.
It works with GNU/Linux, plug and play.
Code:
card 1: DAC [SMSL M-3 Desktop DAC], device 0: USB Audio [USB Audio]
Subdevices: 0/1
Subdevice #0: subdevice #0
Update: The VE Monks+ are overall a bit better with the amp. The sound is fuller (the low end is definitely not the best but the sound has more body with this amp) and the mids have more detail, but not that much, it can be the placebo effect.
I have also tried my SRH440 with the stock pads and the mids and the low end are more clear, less congested.
2nd update: I recently bought a pair of Fidelio X2 and there is no floor noise at all. The channel imbalance have also disappear because they need a bit more power than the Shure. Being open I think it helps too.
Great setup and a fun sound for less than 400€.