The VestAmp can be used as many different things but how ever you use it, it does have a overall sound signature and that is neutral, very wide, very detail and pretty extreme in the treble. In fact synergy with bright headphones can be rather poor because this makes them so sparkly they sound too bright and fatiguing. This is the only really problem because the amp is actually more detailed than the Objective 2 that I have and is perhaps even wider in the soundstage. The background is rather black and silent when being used as a DAC and it does not make any frequency feel lacking and the bass still has oomph if the headphone and music does, it just does not boost it. For smoothness it is a bit edgy especially in comparison to the Objective 2 that is smoother.
As a portable it does do well, I especially like it with a warm IEM as it makes them sound very detailed yet still warm. A portable amp is one of my favourite uses for it, reminding me heavily of the Pico Slim from what I remember of it, yes it is larger but it does have a DAC.
As a DAC/Amp from a computer it sounds less detailed than as just an amp but also a lot smoother, it does sound very good and on high gain it can pair nicely with most things, even though very small. This is probably the worst way that I think you can use it though because the sound is much better in the next way I use it. However it still does produce a good sound, just I feel we do lose detail on this method.
My favourite use of it is as a DAC with a line out feeding to the objective 2, it sounds amazing combing the best of both devices, black background and transparency of the VestAmp and the smoothness and neutrality of the Objective 2. They pair very well and they really do sound awesome.
Conclusion:
This is very good sound and transparent amp, what a solid state is meant to be. However it doe shave its problems like with bright headphones or headphones with peaky treble, the Sony MDR-EX1000 for example is a no go with this but then so is the much loved Pico Slim. The main problem that I have with this however is the volume knob and no matter how silly its sounds, it does make using this hard work on the go even though everything else is perfect for portable use. However the sound is great and I love using it as an amp and a USB DAC!