What amp have you used with the Mixamp?
Im trying to get my hands on a Fiio E17 at the moment and will test it with the mixamp. It just sucks that my M-Stage alters the positional queue. It makes the sound appear closer than they should be which throws me off and hard to tell where the sound is coming from anything that is nearby. Very weird.
With the Mixamp I've used the amp in my Aune T1 and Schiit Magni. I've since passed along the Mixamp but I've used the E12 and iCan with my Recon3D USB.