Did some basic measurements as promised, and here are the results: headphone-out output impedance is 0.7ohm, line-out output impedance is 124ohm. Low gain headphone-out max voltage 1.079Vrms, mid gain 1.75Vrms, high gain 2.655Vrms . Line-out FR cure is flat. Headphone-out FR curve has some minor bass roll-off due to output capacitors. With a 47ohm dummy load, it is -0.75dB @ 20Hz with corner frequency @ 8Hz. Output capacitance is estimated to be around 400uF. Purely on RMAA result, LO looks cleaner than HO (which is expected of course), though both don't measured quite as good as X3, especially on IMD. Since they are still well below the 1% threshold, it doesn't seem to be of any major concern. Output power is quite good, comparable to most standalone portable amps.
I have a 3w desktop tube headphone amp that I use with my HD800s. As I was listening to this endorphination combo, I started grazing this thread, catching up on people`s posts, and came across ClieOS`s interesting information above (thanks!). I see the 120 ohm figure from lineout and think, hmmm.. Reach for my dx50, turn the volume way way down and plug the HD800 in to the LO. Volume up - sounds not bad at all. More volume - great. Put on 24 bit electronica - wow. Just then I get a IM from a friend and we start chatting, me with the HD800s on my head. When that finishes I forget all about the dx50 and go back to web surfing. 40 minutes go by and several times during the album that`s playing I am distracted, as regularly happens, by how shockingly good the HD800 is out of my desktop tube amp. So engaging, so articulate, growling/tight bass and crystal highs.
And then I realize that even though the tubes are glowing, this HD800 eargasm is coming from the dx50. . .