Because there's so many blanket statements being thrown around about the O2 and amping in general, and there's a couple wrong ways about looking at things.
First off, the O2 is a good little amp for its money, and when it was released as an open-sourced schematic, it had a clever campaign behind it through the use of measurements to brainwash people into thinking it's the end-all-be-all in this so called unicorn called transparency. This is just plain bs. "Clever campaign"? It sounds excellent subjectively, and objective measurements support it. The price is so low that there's not enough profit to make out of it. If at all, it's a clever charity program for price-conscious audiophiles. Oh yeah, it's a clever campaign against snake oil amps. Just like other amplifiers that measure with amazing numbers that are all well below the audible threshold, the O2 has its own sound. This is bs, again. Laughable actually. Because this argument is true even for the "higher amps". So we shouldn't care about numbers, which is the only thing constant? Looks like justification for snake oils. It's still a relatively neutral amp, but slightly on the thin and bright side, Another bs. It's not thin even slightly. It's the only amp that brought out bass to sub-bass potential in my HE-400 and LCD-2.2 without losing the details from bass to mids to treble. The O2 is bright or thin only on bright/thin HPs. Even the HE-400 loses its brightness with the HE-400. C'mon. and there are other amps out there-- both solid state and tube alike-- that all vary from warmer to brighter in signature. Just because the opamps within the O2 allow its distortion numbers to be a theoretical .00001 compared to another company's .0001, or just because its noise floor is a theoretical 130db as opposed to another company's 120db doesn't make it the more transparent amp, and you surely won't be able to attribute sonic differences to those numbers alone. Bs. So we shouldn't look at distortion numbers? Another justification for snake oils. And it's not numbers alone, if you notice, the discussion recently didn't even mention numbers. All discuss how it sounds and transparency.
There's two unavoidable facts at play. One, is that every thing about the amp design plays a role in its sound to very subtle degrees, such as the circuitry and its layout, the topology choice, the choice of power, etc; the O2 is no exception in this case. Jason from Schiit Audio has a very good post about this subject on another forum where he goes into detail about the thought-process behind the design of their Statement amp, the Ragnarok, but I can't be troubled to find it right now. Second, is that the conventional measurements for which you see on headphones is limited and often times can not tell a whole story. The same is true for amplifiers. Giving distortion numbers for 3 different tones set at say: 100hz, 1khz and 10khz is miles simpler and easier of a signal than given a complex song with tons of different tones being played at the same time. The science behind measuring isn't quite there yet, and thus measurements should only be taken as a guideline to tell if something is completely wrong, and only that. We shouldn't be chasing for extra 0s after a decimal point and blindly thinking that leads to good audio. Pardon me TMRaven, but this really looks like bs. You justifying lower numbers in other snake oils, actually.
As far as some ideas that are the completely wrong ways of looking at things, I'll tell you this. Tube amps don't necessarily have to be colored, there's no universal rule about that, they come in all different kinds of sounds, just like solid state amps. Just because they measure at .01 distortion compared to .001 distortion doesn't make them colored and flawed. So if an amp has bigger distortion numbers it is not more colored? Many people prefer tube amps for their clean sound and detail over lots of other solid state amps, not because of their color (don't get me wrong though, plenty like warmer or brighter amps in general too.) Tube amps for cleaner sound? If at all, why it sounds cleaner is because some details are lost. Distortion numbers on tubes are WELL-SETTLED to have more noise and more distortion. Now this is looking like a clever campaign for snake oils, TMRaven. Perhaps the worst offender though, is the assumption about using amps to somehow 'improve' a sound. That's the wrong way to go about high-fideility thinking in general. Audio is only one thing: you have an ideal reference-- for this example let's refer to it as a 1, which will always be a whole, every part of your chain down to your headphone will always negatively impact that reference, making it less than ideal. Better designed gear will mitigate that negative impact so it's closer and closer to the 1, perhaps say .95 as opposed to a .92. You will never be able to get over a 1. Now this I agree on.
I'm a practical guy, and I did have the O2 at one point. I would have kept it if I thought it would be the best solution for me, but I've since moved on from it, and to get back onto the topic of the HE-400 I can tell you right now that O2 (and likewise its ODAC) is slightly thin throughout the midrange, which doesn't play into the HE-400's weaknesses too well, but it's still a decent amp for its price. At the same time, the HE-400 doesn't even really need super-good equipment in the first place, because it has a low ceiling of scalability. It has distortion problems within the midrange, and a colored enough sound throughout the treble, meaning upstream gear won't impact it as much as they could, because it will always be imbuing its predominant sound over everything else. It isn't as much of a 'chameleon' as other headphones, like the Sennheiser HD600 or HD800. Not my experience. The bass and sub-bass SUBSTANTIALLY became more present with the O2. The bass becomes enveloping, showing out the real sound signature of the HE-400. It's an entirely different phone from an iPhone 6 to an O2. Colored mids and treble, maybe, but definitely changed by better amps.
So in short, would I recommend the O2 for the HE-400? Yes, I think it's a good amp. Would I recommend anybody explore further up the line in amps? Definitely-- just maybe not for the HE-400. What would other amps add to the O2 aside from coloration?
Lastly, this is a thread for the HE-400. It's cool to recommend and talk about how amps play with it, but it's not fun to see parroting and sheeping over amping philosophies that come off as shallow blanket statements. Talk like that can be reserved for the sound science sub-forum. Somebody with HE-400 asked advise on amps and O2.Why not give advise. There's even no bashing of other amps in here. Too bad praise for an amp you seem to not like at all. Why? I don't see any blanket statements. They're from actual experience. If you don't want blanket statements, we would have to resort to numbers, which you also said are not reliable.
