Shoot, I wrote a lengthy initial impressions message and I accidentally went into Chrome developer/debug mode and the whole thing disappeared...
So, here's a much shorter version. Compared the NFB-1ES with the three Ref series DACs through the Master 5 driving HD-800 headphones.
1. Ref 7.1 sounds full, lifelike, with the widest soundstage and great bass texture. Compared to other DACs, this makes me feel like I'm actually at the performance, whereas the others lack the depth and consequently sound like recordings.
2. NFB-1ES and Ref 7 share a similar tone, sounds nasal and compressed in comparison to the Ref 7.1.
3. Surprisingly the old Ref 1 is closer to the Ref 7.1 in tone and therefore things sound more real and natural, but the Ref 7.1 surpasses the Ref 1 in soundstage, bass depth, and overall fullness of sound.
4. NFB-1ES's bass is especially poor compared to the PCM1704 DACs. Quantity wise it is lacking; more importantly, however, it sounds monotonic and uninvolving. There's something odd about the decay, I think, that makes it sound unnatural. On the Ref 7.1, deep contrabass notes or bass drum of orchestras sound very real and lifelike, but on the NFB-1ES, they sound muffled and distant. Then I thought, perhaps on electronic/synthetic music, the NFB-1ES may fare better. Turns out, even with electronic music, the PCM1704 series still outdo the NFB-1ES in terms of impact and smoothness of bass notes.
5. Not much stood out in terms of difference in treble. Not sure if this is because of cheap cabling. The midrange tone and timbre were very apparent, though as mentioned above.
6. Perhaps I thought the "fullness" of sound and the stronger bass response of the Ref 7.1 was from a higher volume output. So, I tried comparing with the Ref 7.1 volume lowered by a notch compared to everything else. However, this did not change my impression of superiority of the Ref 7.1 in tone/timbre, soundstage, and bass depth and texture.
7. So far, I would rank the DACs in the following order: Ref 7.1 > Ref 1 > Ref 7 > NFB-1ES.
Sorry, I wish my earlier impressions post survived the web browser disaster...