Just picked up a Magni to use with my HD600's and Grado SR60's. Out of the box the Magni is a great match for the Grado's. With the HD600's its sounds a bit cold and steely. Without trying to sound overly critical, I'm hoping that the sound fleshes out a bit more over the next couple of days.
The amp still provides an amazing amount of detail, depth, and musicality for only $100. It's much, much better than the Project Head Box I tried out last year.
What's the breakin period for the Magni?
Not sure if it was the Magni or Modi (as I got and tried them as a pair), but I noticed a large change after a couple hours. I actually immediately listened to them out of the box and thought they were rather shrill. So I left them playing for an hour or two while I went to watch Doctor Who then came back and listened again.
I couldn't say how long the change took, it might have very well been after 10 minutes, but it was at most a couple hours. Beyond that they sounded similar enough that I couldn't say with any confidence that they changed any further in the next few weeks until I sold them.