FiiO E12 with Custom Project-H (Objective 2 w/ Burr Brown op amp and Cirrus Logic flagship DAC)
With the DAC being the CS4398, the FiiO E12 does a good job up against the O2. The O2's vocals are more forward and louder than the E12's. The depth on the E12's vocals are also much better. The seperation and soundstage of the mids(instruments) is also superior on the O2. The O2 is just a bit sweeter than the E12. The E12, I would say is a bit colder than the O2. Of course, colder does not equal more neutral. The O2 would be the more neutral amp, but the O2 accentuates more parts of the song than the E12 does. Now you are thinking, how can the O2 be more neutral if it accentuates more things? Well neutrality is a word that goes into the best reproduction of music and how neutral will theoretically reproduce the music better. And the O2 does reproduce it better than the E12. The overal transparency on the O2 is also better than the E12. However, the units are still nice and closely matched. I'm quite surprised how close they are. To some, the distance between the two may not warrant one or the other, and to some, the distance is huge(purists). But whatever it is, I think that the E12 does a very good job against the O2. It's only that while on the go, I personally prefer a more warmer sound than what the E12 offer.