Can't speak for the other two amps, but I love my Headroom desktop. Very precise, if it's there you will hear it, but lots of depth and fun as well. Don't know about the K701's, but with HD 650's and SR 325i's, it's incredible with both. Mind you I only have the home module and dac, I can imagine the maxxed stuff is that much better. I will be upgrading to the max package in the next 1-2 months.
What really sold me on this amp though was footprint/connectivity. It sits comfortably on my desk and this includes the desktop power supply. Right now I've got my Oppo connected via analog for SACD and through the optical for everything else. I've got it connected to my soundcards coaxial out, and my ipod is on the other analog input. Yes, 4 things connected, all controlled with the flip of a few switches.
Add to the great sound, the excellent customer service Headroom provides. They are very knowledgeable and will go out of there way to make sure you get what you want. The cherry on top, the 30 day return policy, get it, listen, if you don't like, send it back and get a refund. I've done it with numerous phones, and an amp, no issues at all.
There's a rant from a Headroom customer for you. They really launched me into this wonderful hobby full bore. Like I said, can't speak for the other two, but I will guarentee you can't go wrong with a Desktop.