Thought I would translate and summarize some of the reviews the amp has been getting in Japan.
First from
kakaku.com (Japanese deal/price aggregator). The AMP-HP gets an overall 4/5 stars:
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マ・マ writes that he's not sure who the amp is tailored towards, or who he could recommend it to. For those who are used to, and like, rolling opamps he would recommend buying the iBasso D2+ at the same pricepoint. The slight bump on the amp comes in for some criticism, but he/she admits that after 3 days one quickly gets used to it. The battery also could be better in their opinion, perhaps deploying the 006P rather than the stock battery - a change which improves the sound quality in their opinion.
So for those who don't mind the hump, don't mind rolling opamps and don't have a large portable collection or want to build one, this amp may be for them. It is the cheapest rollable portable (in the Japanese market) and for not very much outlay the outputted sound can be changed quite dramatically.
The stock MUSES8820 is a little boring (in their opinion) and the resolution merely adequate
MUSES02 lets the top end soar in comparison to the 8820, and tightens up the sound, but at the loss of some emotionality in the sound.
LME49860 is their top recommendation. Separation is good, and the sound becomes clear-cut/ 3 dimensional. Overall, clarity and detail are improved, without any feeling/impression of "edginess"
LME49720 is grainier than the 49860 and not worth buying over the 49860
OPA2134 is very energetic and lively, but because of this it may not suit all kinds of music
OPA2277 is more restrained than the 2134 but doesn't suit the AMP-HP very well
So even with the drawbacks re:shape and battery and stock opamp he/she does like it very much.
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Kenzo Konoike was using the AMP-HP to power the Shure SRH440 (44 ohms) with the iPhone 4 headphone out as a source. Compared the sound through the amp to the iPhone jack the sound is clearly better in quality, and the driving power of the amp shines through. The lower end of the Shure is extended (about an octave) but in a pleasingly composed and assured manner.
When running the SRH440 out of the iPhone, the lower end does bleed into the midrange especially on songs with a driving baseline, but the amp does a good job of improving the separation of the mids from the bass. He also notes that the amp may match better with phones of higher impedance as the gain is a bit much for the SRH440.
As for opamps, the MUSES8820 is quite good, but in comparison the MUSES02 is a little more mature, but lively in terms of sound (he writes the sound gives a feeling like water bubbling energetically out of a pipe). The lower end does extend down further and vocals in particular sound more harmonious/melodious. The soundstage widens out nicely, with improvements in positioning and location of instruments, and so the overall "live" quality of the sound gets better.
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Hope this has been helpful!
