I got mine in today as well. I have just been using it with the ac adapter plugged in. Hopefully you aren't supposed to fully charge it before using it.
The pk1's are really comfortable compared to my im716s. It's like they're not even there. I think they need to burn-in to get the best out of them, so detailed impressions to follow. Right now, they definitely sound good, but they are a little light in the bass coming from my Koss KSC35/75s.
The Minibox-E was smaller than I expected, even after seeing pictures. It's like a deck of cards, but thinner. This is my first headphone amp, so I am still trying to appreciate the subtle differences between this and the headphone out of the iPod I had been using. I'm using a Pocketdock line-out (maybe an upgrade in the future) with the included mini-to-mini interconnect.
I have tried the amp with all my Koss phones including: KSC35, KSC75 and A250. They sound great with the Minibox-E, and I like the effect the PtoS switch has on them, especially the KSCs. I'm still not sure whether the change is for the better on the A250s.
Next I tried out the im716 with the Minibox-E. They sounded smoother and less edgy to say the least. At first I didn't notice as much change in the bass as I expected compared to the iPod headphone out. I decided to try the PtoS switch, and immediately the bass sounded much fuller. The change could be described as going from only hearing the bass to hearing and feeling the bass.
Now all that's left is to let the Minibox-E and PK1 burn in together. I'm pleased already, so any positive changes over time will just be icing on the cake!
-CK