I've never said in any way that I don't know anything about USB power. I'm quite well informed on the subject. I don't know why I'm bothering to reply, because you obviously can't read since the statements I've made are supported by the links several people have provided. I hardly had to "desperately search" for source material when I quoted something I had referenced previously requesting you to read it and that blog article was linked on the first page of comments. The fact you STILL haven't read and understood the source material you claim supports your position, and the fact you're even arguing with me at all considering I assume we're in agreeance on the fact the ODAC is an acceptable DAC makes me wonder what your deal is.
Quite frankly expecting electrical measurements to support my position is absurd. It's not like I have a dScope III laying around the house (expensive equipment) to generate measurements to post. I seriously doubt you do either. Not only that, as I've stated previously, I don't currently own any of the hardware we're discussing here, but I have had the opportunity to extensively demo it providing me the first-hand experience necessary to make a judgment. So, even if I had the equipment I would be unable to provide measurements of my own.
You're also SIGNIFICANTLY misrepresenting my position in your reply. I said in the ODAC I don't hear any of the tell-tale noise that I associate with USB self-powered DACs. It's EXTREMELY apparent in almost any USB self-powered DAC on the market when you listen without music playing and with your amplifier volume control set slightly above the minimum in a quiet room. Every DAC has a noise floor, and where that is impacts the dynamic range that the DAC can produce when it generates the analog signal. When you have power noise exceeding the noise floor of the DAC it reduces the overall dynamic range that can be achieved in the signal chain. It also has the effect of introducing that noise directly in the signal, which means the noise can be amplified just like any other portion of the analog signal. This is exceedingly undesirable. Hearing power noise, or the harmonic distortions produced from it, does not require golden ears. Almost anyone can hear it.
I readily admitted the one mistake I made earlier, which was a faulty assumption that the ODAC+O2 combo unit (the only one I've heard, I have not heard the ODAC standalone, although they are equivalent in theory) used the input power from the wall-wart to drive both the amplifier and DAC stages, rather than using the power from the USB for the DAC stage. The reason I had made this assumption was directly because of the fact I did not hear noise I would expect from a USB-powered DAC when utilizing it. If anything, you should take this mistake on my part as a boon on the part of the ODAC, as it's certainly the only USB self-powered DAC I've heard that doesn't fail in this regard.
Regarding the line level output, there's 3 basic standards to care about. In purely analog audio, there are two, one intended for professional applications, and one intended for consumer applications. These typically aren't rated in vRMS , but rather in dbU or dbV respectively. Regardless, you can do some math to get a conversion which results in a nominal vRMS of 1.23 (rounding up) or or 0.45 (rounding up) respectively. These are less interesting to me, as these are intended for use in recording and analog playback respectively. However there is also the Redbook standard which describes the CD Digital Audio format, including several specifics of the process of converting the digital signal to analog, and what that line level should be. Disregarding for the moment whether you agree with the Redbook standard, the standard specifies a nominal output voltage of 2vRMS for the analog signal. Per the Redbook standard, as I noted. 1.5vRMS /IS/ a weak output signal for a DAC. Disagree with it all you want, but the statement I made was factual and in accordance to the standard.
As to what the difference in the output levels means, well it has to do with gain characteristics. Since I primarily listen in an environment where I originate using digital audio, typically ripped from CD or SACD, encoded in FLAC and with the end result of driving either high impedance (600Ω typically) or power-hungry orthos, I have a set of things I look for when demoing equipment. By driving the line input of my power amplifier with a weaker line-out signal from the DAC, it has a few immediately obvious effects that allow me to note this without even requiring measurements of the output voltage. The primary obvious thing is that I will have to drive my amplifier harder by increasing the volume level past where I would expect it for that particular test track to get the same volume output level. By requiring an increase in input gain to achieve the desired volume, I also have a higher potential for introducing noise into the signal. If the signal is weak enough, it requires a separate pre-amplifier prior to being connected to the power amplifier, which does introduce additional gain but does so in a more controlled manner. I've yet to demo a DAC that required an external pre-amp to drive my power amplifier, however it would be necessary off of most analog equipment such as a tape deck or turntable, simply because they produce a significantly lower line level signal. This is one of the reasons why you typically see someone employ a "phono pre-amp" combined with a turntable, before introducing the power amplifier.