Pros: Feels well built
Cons: Removes detail from music
For testing it, my source is a Xonar D2X using foobar 2000 w/asio. I'm using HD 600 headphones, a FIM audiophile reference disc and McIntosh audiophile reference disc.
Without this amp, with the HD 600s straight into the Xonar, music has a lot of clarity and feels very precise, it's easy to appreciate the sound. The xylophone in Canon in D - All Star Percussion Ensemble on the FIM disc sounds very clear and very distinct, it is easy to pick out the separate instruments and there is a lot of fun listening to everything in this piece. The violins in Starker plays Kodaly from McIntosh Audiophile Test Reference #02 sound beautiful. You get a lot of detail and it almost feels like you can see the strings.
When going thru the amp things change. The audio quality lessens and the clarity in the top range has disappeared, the highs aren't there like before. The xylophones don't sound as real nor distinct. The instruments blend in together a bit so it's not as easy to separate them by their sound and enjoy each one individually as it is without the amp. The violins in Starker Plays Kodaly on the McIntosh cd sound plain, they've lost the luster and detail and there is no distinct part to focus on.
The Head Box brings out more lows in your music so you hear more/lower bass which benefits rock and beat heavy music. I did some testing using a Christina Aguilera CD (Stripped - Beautiful) and the bass is fuller and has more attention with the amp but the audio quality suffers and her voice lost detail. It blends in with the instruments so it's not as easy to focus on.
I would recommend this amp for people who listen to tracks of wind blowing or tracks of people stomping their feet. If your headphones or source are low quality then you have nothing to lose and this amp will do it's job. If your headphones and source are high quality then you don't want this amp, the loss of quality makes it unusable. The more quality in your setup, the more you'll not want your setup to be brought down to the quality of this amp.