Heh, heh. I know what FlySweep is going to write...
imackler, I've expressed my opinion to you privately, but for the benefit of others...
Let me start by saying that I like the O2 very much. I think its a great amp, very neutral, plenty of power, decent build quality, good price. But in comparison to Neco's amps:
Problems with the O2
1. Sound quality lags behind the Neco, which is just phenomenal in terms of sound quality. My old Neco portable (the pre-cursor to the current 18V v3 amp) is what led me to sell my Burson HA-160D. I'm not kidding. In particular, bass on the Neco is superior to the O2. (Not that I generally found bass on the O2 lacking, but it is definitely noticeable compared to the Neco.)
2. Battery life is pretty poor in practice, which I think is partially related to the next item.
3. No automatic switch to turn off recharging when battery is full.
4. Not exactly small.
5. PSU is OK, but because the O2 can't be charged over USB, it's still an annoyance when traveling.
6. Build quality is good, but not up to Neco's bomb-proof standards.
The new V3 Neco gives me all of the following advantages:
1. Great, great sound. In particular, you need to hear a Neco pair up with a pair of HiFiMAN HE-500s. It's like they were made for eachother. The Neco is phenomenal with other cans, too, but it's mind-blowing with the HE-500s.
2. Smaller size than the O2
3. Available bass boost if desired
4. Much, much longer battery life (with lithium ion battery option)
5. USB recharging (if desired)
6. Automatic recharging stop when battery is full (with lithium ion battery option)
7. Neco build quality
The O2 is a great amp, but the Neco just offers a ton in terms of sound quality, features and build quality. For anyone who is curious, Neco only sells his amps on eBay under his seller name "Neco Soundlab" or "Neco_soundlab", I forget which. He is a true undiscovered genius. Everyone I've recommended his amps to has subsequently written to me and raves about them. I have one of his BOSFET amps as well, which I use with my HiFiMAN HE-6s, among others.
Best,
Adam