Just got my little magni in the mail!
For those of you who are perusing this thread to determine whether the magni is a little too much for your sensitive cans, you can lay your fears to rest. I'm using it with my HE-400s and my ATH M50S, both fairly efficient headphones, and you won't have a problem. With both of these pairs, I sit comfortably at around 9 or 10 o'clock with the computer at 100% volume (I'm using a xonar DX as my DAC). I don't notice any substantial channel imbalance above 7:30, and even below that, you have to really strain to notice it. When the pot is all the way down, I don't hear a thing.
It also sounds great! Crystal clear with a nice improvement in instrument separation and a nice, round stage. Can't speak for the O2/ODAC, but HE-400/ATH M50 owners can't go wrong with the magni.
If anyone wants I can give more detailed impressions after I listen some more.