LOL my definition of transparency is if I can use it for 2+ years and it doesn't audibly change the sound of any of my headphones.
If the amp is only good with specific types of headphone then it's somewhat of a fail for ME. To me a headphone amp should be equally good with those that are bright/warm/v-shaped etc.
I'm sure others have a different idea of what "sounds" transparent to them.
For me the most transparent amp i've "heard" is the Micro Amp with Astrodyne unit. Usually when people criticize it I think it's their source. O2 sounds pretty similar..as close as i've come. I'd probably fail a blind A/B test perhaps. There's some small differences though.
BTW I read somewhere that the O2 can sound a tiny bit more trebly than normal with a weaker AC adapter using the Q701. Doubt this is true but it's worth a try.
ODAC mostly definitely sounded transparent. Didn't sound bright/warm etc. No change to any of my headphones. Same with the Modi.
Edited by tdockweiler - 9/19/13 at 5:09pm