Hi Bowei / Hi Rik
Bowei - I'm going to post here instead of the PM. Reply would be the same - this way others get to see it.
First up - I'm a self-confessed "neutral head" - with a slight preference toward a bumped or forward mid-range. I like detail, but not if it's presented at odds with the rest of the spectrum. I like nice bass punch - but am not a basshead. My main genres would be classic rock, progressive rock, blues, jazz, female vocals, bluegrass, classical, electronic (esp triphop), opera, and even some folk - love a lot of indie stuff. I listen to the very occassional bit of dubstep, hardly any rap or heavy beat based stuff. I don't like heavily V shaped cans (DT990 M50 etc) - and you'll see by my sig that most of my cans (apart from the 325is) are quite balanced (DT880, HD600, SE535LE). I'm 45, suffer from permanent mild tinnitus, and because of this need a little high end bump/sparkle to really enjoy my music more fully - ie I do not like overly dark sounding gear. When I bought the HD600, I did so after hearing the HD650 - HD600 just had more balance and a little more top end.
OK - that should give you an idea of what I like - we share some genre interests, but it does appear you are more into pop, and beat based music than I am (probably expected given our age difference).
I've formerly owned few dacs / amps - main combo before the NFB-12 was an E7/E9. I thought the Fiios were good at the time - but wanted to try something else - glad I did. The NFB-12 made me realise how thin sounding the E7 was. Yes it might measure well, but it's lacking body / texture / fullness - which the NFB-12 has a lot of. The NFB-12 is also quite warm (yet not overly dark) which does tend to suit my style.
**Next part is all IMO - others may hear differently**
The nicest part is the filters. With the default 2x, it's overly warm, syrupy, dark and the detail is rolled off. When I first bought it, my immediate thought was that I had purchased a lemon. Once I changed the filters, I then realised how much you can tailor the sound. In my review I listed the two filters I really liked - the 8x has a real increase in detail, while the 4x is a nice mix between clarity and warmth. I wish I had the 12.1 - the ability to switch between filters on the fly would be great. Is it worth it for another $15 - yes absolutely IMO.
As far as strengths of the NFB-12 go - good dac (plenty of body), very powerful amp (600 ohm no problem), good form factor, great price, configurable through filters, (so far) appears to be a very good company to deal with. It also has great inputs (USB, optical, coax - I use coax), plus otion to output using DAC only to powered speakers or to another amp.
My intention was always to use the NFB-12 as a stepping stone - but lately I've started wondering if I really need to change it at all. It's doing everything I want - and I'm actually completely satisfied with my audio set-up at the moment. In fact I can honestly say that I'm far more interested in getting more music now rather than gear. Yes I'd like to hear other amps / dacs (curiousity) - but for now, the NFB-12 (with my set-up) ticks all my boxes.
Re - tubes. I purchased the Little Dot MK IV from Rik (Tilpo), mainly because I've always been curious regarding tube amps, and - because I particularly like a slightly bumped mid-range - because I've wanted to experience that 'euphonic/euphoric' tube sound. **BTW - plug for Rik here - travelled 1/2 way round the world and arrived in immaculate condition - great person to trade with**. I have to say that initially the difference in sound was very small - not the change i weas expecting. Possibly because the NFB-12 is full bodied, warmish and well textured already. Anyway - I bought some tubes to try rolling. The ones I wanted were the Mullard M8100s - which are supposed to have areally euphoric warm mid-range. I have these running now and the difference is more apparent. I really like the sound - and once again, totally satisfied. The difference is still not the famous "night and day" that is so often quoted on these forums. But it's enough for me to still use it almost every night.
For my preferences I've reached my mid-fi end-game rig. Yes I'll probably get the itch to climb the ladder again in a few years - but it'll be once the kids have left the nest, and we have more funds available. Until then - I can thoroughly and unreservedly recommend the NFB-12.1 for a value desktop solution.
BTW - I also read Mike's review. My take on it was that he didn't like the default settings and it coloured his view right from the start. I've heard it and thoroughly explored the settings with my cans. If it's not a good amp according to him - I couldn't give a toss. I love it - that's all that matters