My Magni 3 impressions after two weeks with it:
My first setup is a Modi Multibit -> Magni 3 -> HE500, Alpha Prime, Sennheiser MX985, iSine20, or RE272, so I'm using it for some very power hungry planars, as well as really sensitive 16 ohm earphones, and the Magni 3 sounds equally good with either, as expected.
It's probably the most transparent amp I've owned (the Fiio e11 is also transparent, but it's nowhere near the quality of the Magni 3, which is ultimately cleaner and more resolving. You need really revealing cans to tell though). The Magni 3 has plenty of power & headroom for anything I own at any volume I'm willing to try. It sounds just as good on low volume as it does turned up; the bass doesn't go away at lower levels.
I really like the volume control: the knob is solid and has just the right amount of resistance, and the way Schiit mapped how the amp responds to changes on the volume knob is perfect for me. I can switch between different headphones, and get the volume nearly matched without thinking about it, before I even plug the new ones in.
My second setup goes Modi Multibit -> Magni 3 preamp outs -> Stax SRM-727A -> Lambda Pros. I haven't been able to compare this setup to plugging the Modi into the Stax directly, but this might be a good test to see whether the Magni 3 has a "sound" or not, but I'd guess not.
What I *can* say is that it sounds great. I've found that at 100% volume on low gain, the Magni 3 spits out a line level signal, which is nice. I've tried it on high gain too, and I *think* it may sound even better that way. But until I'm certain the Stax can handle that kind of voltage I'm not willing to try it for very long.
Using the Magni 3 preamp outs for this purpose has made it easier than ever for me to switch from one headphone to another.