I can't really tell the difference between the HE400 directly into an iPad2 and the HE400 with the ODAC/O2. To give you a reference point for my hearing acuity, I can tell the difference between 24-bit, 96khz Nine Inch Nails - The Slip and the 320kbps, mp3 version. You can download it for free here: http://theslip.nin.com/
I don't get your point. Are you saying that the iPad2 has superior dedicated audio amplification, or just audio quality on par with the O2/ODAC? Because your comment seems like more of a knock against an amp than against a headphone.
The iPad doesn't have close to the power of the O2 either, so 'loudness' should be the most obvious difference here.