Figured I should copy my post from the HE400 thread here.
"I have a Magni and HE400's in use as I type. I don't have much to compare it to, only the E9. Compared to the E9 it is definitely better with the HE400's in my opinion. Everything seems a bit more clear and the bass has a nicer punch to it. Not really sure how to describe it, but the mids seem less congested on the Magni than the E9 as well. Only have about 2 hours on the combo and have only side by side compared for a short while, but these are my early impressions. It's not a day and night different, but I do definitely hear a nice change :)."
Also since then I tested my Heir Audio 3.A customs and my Sony Mh1C with it and if you are looking at this amp specifically for IEM's I would look elsewhere. I had to keep the volume at about 8 o'clock and it starts at 7 o'clock. Not much room to adjust volume at all and a minor slip and you might hurt yourself haha. This thing is probably too much power to realistically use IEM's as a primary amp for them.
Might post some more impressions later with my modded T50RP's if I get around to it.
Also regarding comments I read earlier about the long volume knob, I agree it is a little long and maybe awkward looking, but it is really nice to actually use. Another random comment while I'm thinking about it, this amp is basically silent. No noise when adjusting volume, and just a very faint noise even when the volume is turned all the way up with no music playing (just did it to test haha). Even with IEM's it has to go past 12 o'clock to hear noise and that would probably blow my ear drums out if something was actually playing.