If you have sensitive IEMs be careful when plugging in the device. Plugging or unplugging the source to the AMP socket will give you a loud-static pop. It's noticeable on gear like the Sennheiser HD600, but it's almost unbearable with IEMs, such as Brainwavz's own S0.
Here, I also want to draw attention to EMI. From time to time you get some really bright static noise. Then thing is, it's seemingly random. When we actually actively placed the device in a "noisy" environment, with multiple PCs running, smartphones ringing and multiple routers routing, there was no noise whatsoever. However, when riding the train it picked up, more than a half-a-dozen times, short and very annoying static.
AP001 delivers a relatively clear sound, though not necessarily a balanced one. You see it has an obvious sub-bass and mid-bass added to the overall sound. Depending on what kind of music you're listening to, and on what gear, it may end-up sounding great or unbearable. For example, when listening to Brainwavz's S0 IEMs, the sound had too much bass for me. It was also added a veil to the earphone which really made it tough listening to it. However, when paired with to brighter IEMs like Logitech's UE TF10 was much better, it had a warm and organic signature, with an albeit boomy bass.
I also used it with some proper over-ear headphones, including the Philips Fidelio L2, and the Sennheiser HD600. Fidelio L2 is designed to be used alongside smartphones, though without proper amplification, there are many a thing lost to the ear. I think HD600 is somewhat above Brainwavz's AP001 power, since there is a very noticeable difference between using a larger-size portable AMP like Fiio's E12 and it.
When paired with the Fidelio L2, it has a clear sound, with a somewhat boomy bass, but noticeable improvement in instrumental separation, soundstage, and imaging.
While the HD600 is a bit over the AP001, the AMP still does its job well enough. The bass is boosted considerably, and some detail is brought up, but the soundstage & imaging is still not as good as it could be.
Overall, the actual amplification by the AMP is relatively subtle, with the bass being most boosted. It adds a nice thump to most headphones, and little to no bleeding to the midrange, which is always a plus. It also amps up the volume (according to Brainwavz, the amplification is about 120%), though I have to say the amplification is a subtle one.
Placed alongside Fiio's tiny E06, Brainwavz's actually AP001 sounds cleaner, with a wider and deeper soundstage. Both mini amplifiers feature bass boost, but E06 has a rocker / switch to turn the boost off.