Did the older ones have trouble with output impedance? I used the 601 and found its output impedance to be perfect for all the earphones I used with no defaults in any one test, listening suite, or otherwise. That said, the HiFiman series use chips that make use of low-pass filters, which roll off the high frequencies - no matter which earphone is used. It's been a while and there have been a few products since the 601, so things may have changed, but the 801 and 601 had no problems with output impedance - at least as far as the units I tried are concerned.
Liking the roll off though, that is up to you. I dug it with certain music.
Not so sure about that Shigzeo. At least from what I've heard all Hifiman 801 units suffered from high output impedance'. Even the one with the game card which was around 18 ohms I think. So there was no unit with less than 1 ohm.