Ok, so I’ve listened to the Nad C 375BEE now a bit with both the 812 and HD 800. As expected the high output impedance boosted the bass compared to the O2 and Essence III that have lower output impedance. It wasn’t as bad as I expected though with the 812. The HD 800 also sounded pretty good, though the Nad has a problem with background hum. With both headphones in quiet parts I could hear humming in the background, and it was actually worse with the HD 800 than the 812. When playing music the hum disappeared into the background.
It’s a good match with the HD 800 tonally. I can see why someone could be happy with this sound, apart from the obvious background hum between tracks. It produced a warmer sound than any of my headphone amps. The warmth was at the cost of resolution and focus in the midrange compared to my other amps. But it wasn’t a disaster by any means, so I say if you have this amp and you like how it sounds with your headphones, just keep using it!
Note that I’m using the Xonar Essence III as a pre-amp for the Nad, so it’s possible the background hum might be reduced if I was using the one the Nad comes with. The way I’ve decorated makes it inconvenient to squeeze in behind the amp to reattach the jumpers between the pre-out and main-in to test. I used to have Dali Mentor 2 speakers, and now I’m using B&W CM8 with a Velodyne sub. I’m very happy with how the combo sounds. The only problem with the Nad when using it with speakers is the built-in pre-amp which uses an idiotic volume scale. All the volume comes in the first third of the scale, so you have to touch the volume very carefully to not blow your ears out. Using the Essence III as a pre-amp is much nicer. Not only because it sounds better, but mainly from a practical viewpoint since the volume control isn’t as crude.
I wonder what is causing the hum. Could be a source/dac issue rather than an issue with the amp.