So I just got my NFB-12 today. Using optical from my PC to the NFB-12 and plugging my T1 in directly, the sound is far better than Xonar DX + LD MKIII combo that I was using in the short term after I gave my Sparrow away. The sound is warmer, crisper, and sounds separation is better. Everything just feels "fuller". I had to remove the equalizer setting I was using in the meanwhile which was boosting the lower to mid bass by a couple dB, and removing a couple dBs from the highs, because it was overly warm after putting the NFB-12 in place. It's just right with the EQ off, I could settle with using this as a dedicated DAC/Amp for my T1, DT990, or DT880. I could definitely see why the NFB-12 wouldn't be too suitable for Sennheisers like the HD580/600/650 as it would make an already warm headphone that much more veiled, it seems like the NFB-11 would be better suited for those as it's a brighter, more revealing DAC/amp (and also less forgiving with recording).
Setting it to High gain on about 10'o clock gets the highest comfortable volume for me on my T1, on low gain it needs to be set at around 3'o clock or more, and even then still sounds kind of hollow.
The size of the unit is quite a bit longer than my Sparrow was, it's actually almost identical in size to the LD MKIII in width and length, just not height because of the MKIII'd transformer and tubes sticking out top.
Tried USB briefly to see if it experienced some issues other people had with noise - no issues, the optical vs USB isn't easy for me to differentiate, but I didn't bother using USB very long.
Using it as a dedicated DAC, mine came with it set to "variable only" so I had to open it up and change the jumper. The gain switch still works in fixed mode, setting High gain is required to get proper output to my LD MKIII (which is also set to it's highest gain setting), it seemingly works out perfectly because it allows the MKIII to sit at the perfect 11-12'o clock on the volume pot for a maximum usable volume.
Anyways I've only used it for about half an hour, so it's hard to really say too much more. Everything is just better, this was the slight warmth that my T1 needed to make them perfect for me again.
Also, here's a picture