RE SR-404/Lambda vs. HD600: the HD600 scales better. Infinitely better. Which is why the many little flaws that it has will start to go away once you get some serious power behind it as well as balanced drive. But, nothing I've heard so far will make the SR-404 into something it isn't, i.e. a very electric-sounding headphone with an unnatural midrange. I can certainly see how you could undo that coloration with a signal path that has the reverse coloration, but I never had the means - or the motivation - to make it work.
RE definition of neutrality: obviously I'm not the OP here, but my definition has 2 parts - 1) relative balance between bass, mids, and treble that is comparable to my ear to a speaker system in an acoustically-treated environment that measures perfectly flat, and 2) an accurate tone to each instrument, which stems from the lack of spikes and dips over specific narrow frequency ranges. The SR-404, for instance, succeeds on point 1, having a very good relative tonal balance in general terms, but fails on point 2, having a very specific spike at 2-4kHz which screws up midrange tone. The HD600 succeeds on both fronts and is a lot more neutral.
RE midbass boost: if you have a headphone that, to a theoretical measuring device, has the same FR as a neutral speaker in a listening room, then to the human ear, it will sound deficient in the lower registers. A headphone's transducers simply cannot move as much air as a speaker, and tactile sensations will be lacking. The tactile element is vital to our perception of bass frequencies, which is why so many headphones, HD600 inclused, boost midbass slightly to give more audible bass information to compensate for the lessened tactile element. I don't see that boost as a violation of neutrality, rather, I think it is necessary, provided that it's not overdone - and in most cases, it is. With a well-driven HD600, though, not only is the bass more impactful than nearly any other headphone, but the midbass boost is lessened compared to a poorly-driven HD600, and is pretty much exactly what you need to psychoacoustically perceive the HD600 as being neutral.
Transparency and detail in my book have nothing to do with neutrality. The HD600 is not that detailed or transparent. The SR-404 is one heck of a lot more transparent and detailed. This is also, to my mind, the reason why so many people equate bright-sounding headphones with neutrality. The ER-4S is not neutral. It is really, really bright. But, its brightness brings out detail that would be a lot harder to hear were it neutral, which gives it the illusion of transparency, which then in many people's minds equates to neutrality. But, in the textbook sense, it has nothing to do with neutrality.
My .02... well, with current dollar inflation, more like my $3,220.
