I've been using the amplifier in my NFB-12 and never seen the need to get a dedicated amplifier. The NFB-12's amplifier is dead silent and has enough power to blow your headphones up. Plus it's got a flat frequency response that extends bellow and over human hearing. I just don't see why I should spend more money on something else.
I can already admit that the music I listen to is the bottleneck of my setup. I just HATE compressed music, but sadly it's the norm in the genres I listen to. Champagne to the day the Loudness War ends.
No amount of money spent on hardware will make over-compressed albums sound better. So unless you strictly listen to "audiophile level" recordings, there's a point where good enough is good enough. The NFB-12 fills that role quite well.
There are some improvements with the O2 but I get what you mean. I was able to get the O2 at a very good price so I got it to do tests and comparisons on. I'll be selling it in the comming weeks. It's a desktop only(no batteries) use O2 customized for high performance purposes(Burr Brown op amp with NJM4556) and uses RCA inputs and quarter inch output.