Pros: Format support, sound quality, amazing build quality
Cons: Front panel easily scratched, low output power
Excuse my English
First off the DAC is 24bit no matter if its on PC/Mac/Linux
it Decodes PCM 192K and DSD 2.8/5.6 MHz
Build quality: One of the best on the market. the casing of the unit is 3mm thick aluminum. Components on the circuit board are neatly laid out. The HP-A4 have its own power regenerator, therefore inconsistant usb power output during operation don't affect the sound quality much.
Sound Quality: I don't have high end headphones, but Low impedance phones really sounds good (<150 ohm). The sound is balanced.
Filters: Fast and Slow roll-off for PCM, Low and High cut for DSD. I can't really hear the difference though, but DSD tend to sound better with the Low cut filter.
There are also other features:
Optical input and output, can be used as a converter. the output is always on.
RCA output, i also have powered speaker that I use with the HP-A4.
Specs from fostex jp website:
32 ohm load: 100mW
300 ohm load: 20mW
DAC chip: Taxas Instrument PCM1792A