Opening - I haven't had time to compare this to my old DAC and Hybrid Tube amp. I will post again once I get to do a comparison. I am especially intrigued to see how the DAC section stacks up against my old one. I am using FLAC files through the coax out of my X-Fi with bit perfect output.
Package - Well packed. The optical cable looks decent with a white braided sleeve covered in a clear tubing with gray and yellow plugs. USB and power cable are generic. Came with 10 jumpers as well.
Build - This thing is solid and well built. Bottom of mine was scratched up pretty bad, but the rest of the case was in perfect condition. I need to get some rubber feet for it though.
Highs - Fairly detailed, but smooth. Doesn't have that decay that goes on forever with things like cymbals. Female vocals that have that Sssss definitely get smoothed out and slightly chopped off. So this might be good or bad depending on your preference. I didn't noticed any difference on High/Low gain.
Mids - Forward and energetic on High gain. Slightly more mellow on Low gain. I think this amp shines in the mids. On Low, it was boring at times, but more balanced with the highs. On High I had to listen a tad more to pick out some details, but this was slight.
Bass - Fairly tight and very deep. Maybe boomy at times. With my HD580 this was a nice thing, with my M50 it was a bit much at times.
Soundstage - Pretty nice, but my initial impression is that it isn't particularly wide or deep. Low gain seemed to provide a tad more depth and High provided less depth, but more width. These differences were slight.
High/Low gain - With my 300ohm HD580 I could max out the volume on Low, but it was still at listening level for most music. On High I only got to 11-12 o'clock. With the M50 there was plenty of headroom on both. Most of the differences I talked about were more apparent with the Sennheiser, so I am not sure if that is impedance related or the Senn being the superior headphone. With either phone, the High gain was more dynamic and played effortlessly and with more energy. I can't imagine ever using Low gain.
Other - There is music playing at the lowest volume, but this shouldn't be a problem for most. Dead quiet through 11 o'clock with no music playing, then some typical amp hiss creeps in, but it is very low in volume, especially with higher impedance phones.
Conclusion - Seems like a solid product, especially at this price point. I'm not sure it is the best match for Sennheisers, but only because of the lack of extension in the Highs. Again, a preference thing. With the forward mids, deep bass and smooth top end, this might be a perfect match for the 880/600. I might have to buy it again to test. Again, these are initial impressions, so I might totally change my opinion after a week or so and after I compare to my existing setup.
UPDATE - A quick comparison to my 13 year old DAC is that the the NFB-12 is congested in the mids and probably not as transparent. Also, comparing the NFB-12 amp to my 3 year old, $90 hybrid tube amp is that the tube amp might be more detailed and dynamic. Sennheisers are known for their synergy with tube amps, so this is not real surprising. More to come....
