OK, now it is time for me to show my non-conformist nature, and to give you all a chance to dispose of old tomatoes.
My listening time recently was split between H10, and vintage NAD 1020 preamp that I have very recently recapped.
And I have some surprises, that I do not understand myself, but ... that is what my ears are hearing and I stand by it.
First, before recap, 1020 was losing to H10 all around. It had nice sound, but it had strange slight coloration to it, no low end extension to speak of, and slightly jacked up highs.
After recap plus minor upgrades and ~20 hours of burn-in, that changed a lot. Overall sound is very reminiscent of H10. Switching between those 2 with variety of headphones (K553, K272HD, DT250, AD900) taught me that on moderate-loud levels they are more or less similar sounding (with AKG K553 sounding a bit better with H10 - deeper low end, mostly). With reduced volume (so called low volume listening, safe for my oh-so-problematic ears), H10 wins easily, maintaining very believable soundstage, while NAD 1020 sound 'deflates' and becomes fairly 2D on lower volume levels.
Here comes the kicker. When I tried HD650 with that NAD 1020 on moderate-loud levels (what I would call critical listening volume), I was floored. This cheap pre-amp has no business sounding that good, even if it is legendary, and is known as best budget pre-amp in existence. It is hard to say what it does better than H10, one obvious thing is its low end extension is either slightly deeper or a little stronger (or both), and detail/texture is a bit better but that is not all. There is strange sense of 'presence' and 'clarity' using this combination, that manifests itself a lot when listening to complicated and loud organ music (with multiple low and high registers building melodies at once), or with Gregorian chant (multiple male voices). I know how organ sounds in the real world, and this combo teleports me to concert hall - when I close my eyes, I feel that I am THERE. Same with gregorian chant - I feel like I am in the same room with monks. Especially when I play vinyl. One word I came up over and over using this combination - visceral. Yes, that is how it feels.
And I can not get the same effect from H10. It sounds great, especially on safer listening volumes, smooth, deep, detailed, and not harsh, but it just does not have that crazy synergy with HD650. Funny thing is, with 250Ohm DT250 (very good choice for low level listening, unlike most phones it preserves low end detail even when played quietly), it sounds AMAZING - on lower volume levels, but when cranked to critical listening levels, NAD starts to take over in a way similar to HD650, just not to such a ridiculous extent.
All in all, I am starting to feel that H10 is shining its brightest with 'lowish' impedance headphones that need a lot of current (or with any headphones as long as you keep volume relatively low!), and while it works well with higher impedance headphones (250 Ohm +), it just does not extract that very last bit of performance from them. Of course, I am extrapolating from small and very limited set of headphones I own, but that is how I feel about it at the moment, because so far MY dataset fits this hypothesis. Also, there was other data posted here suggesting great synergy of H10 with planars and K7xx, and it fits that theory as well.
There. Those days when I want to listen to something critically (as in - do nothing else and enjoy music to its full extent - for limited time), I reach for HD650 + NAD 1020. If I just want to listen (relatively) quietly and enjoy music for long time (and can't use my good speakers for some reason), it is H10 with DT250 (best for really low volume), or any other pair of phones. If I had planars or K7xx would likely use them with H10, but that is purely a speculation.
Got those tomatoes ready?
. And thanks to you, bahamot, for posting it.