I posted some time ago how well my two Pioneer AVRs do powering my HD800. My first exposure, by accident, was when my friend brought his Violectric stack (V800 and V200) over for me to listen to. Towards the end of the day, I asked him if we could bring them down to my basement and compare them to my home theater receiver (Pioneer Elite SC-25) just for grins. Using my Squeezebox Touch as a "transport" we volume-matched as closely as we could with my RS meter and then started listening.
Now my friend is a professional recording engineer who is much more critical than I am, and he swears that with certain parts of the frequency, he could hear a subtle difference between them. But he admitted that he was shocked by how close they were - so close that he admitted that if he were blindfolded, he didn't know if he could reliably identify one from the other. As for me, they sounded identical to me.
This led me on a quest to find a new receiver for my 2-channel system with a good built-in headphone stage. I believe it was here that I got advice about high output impedance acting as a voltage divider, so over the next couple of weeks, I visited a number of audio stores with my Denon D7000's and Sennheiser HD800's to use their relative output volume as a rough guide to finding an amp with low output impedance.
Long story short, after sampling numerous receiver and integrated amps (Pioneer, Yamaha, Sony, Rotel, NAD, Marantz, Onkyo, etc.), I discovered that only the Pioneer receivers had a low output impedance. I picked up a Pioneer VSX-1122 after these trials and am very impressed with it's headphone output. At low to medium volumes (where I do all of my listening), these Pioneers sound exactly the same as any well designed SS headphone amp I have heard. They are not lush, warm, or wet-sounding by any means. Actually, they really don't have any signature at all - they are remarkable only in that they are completely unremarkable.
I'm certain that they have a basic opamp headphone stage and are probably power-limited with high impedance headphones like the HD800, but this isn't a problem for me as I don't listen at high volumes. I have listened to some high dynamic range recordings with my VSX and so far have not detected any clipping or harshness with transients. Overall, I would say that these Pioneers are easily comparable to basic no-frills headphone amps like the Objective2 or Schiit Magni.