Hi, I have the gains set to x2.25 & x3.4, the 650's are well driven and there's no sign of that vail...
I can only be subjective about the performance of the O2 as I've not much experience of other headphone amps: Denon AVR-2310 [home cinema amp], CMoy [various opamps] and a Asus Xonar Essence STX [headphone out only]...
For me the O2 is a transparent amp and what I'm hearing is very much that of the 650's signiture. IMHO it sounds a bit like the standard STX but with better HF responce and deeper, cleaner, well controlled LF's. Overall it's got a well balanced sound, slightly warm [650's], with detailing and sound stage that are beyond anything I heard so far. For the price it's an amazing amp and well worth the money, how it compares to the likes of The Wire, Beta 22 etc, I don't know.
Hope that's helpful.
Edit: I've listened to 40-45 albums this last week, with a minor few of the cd's I've thought the amp sounded good but wasn't doing the material any favours, revealing everything. With the majority of recodings I've been completly blown away by it...