http://www.head-fi.org/a/describing-sound-a-glossary
See "harsh," "hard," or "piercing" - and note that I haven't actually heard the HiFiMan headphones, I'm just going on comments from the CSD thread.
Like here:
http://www.head-fi.org/t/566929/headphone-csd-waterfall-plots/120#post_7736210
Regarding the bass, this is just the general trend I've read related to Orthos - it's not a "bad" thing; many people are happy with their Audez'e, HiFiMan, and Fostex parts - I personally don't like "lots of" bass.
See a comparison between FR graphs:
HE-500
http://www.innerfidelity.com/images/HiFiMANHE500.pdf
HE-6
http://www.innerfidelity.com/images/HiFiMANHE6.pdf
vs
HD 650
http://www.innerfidelity.com/images/SennheiserHD650.pdf
SA5000
http://www.innerfidelity.com/images/SonyMDRVSA5000.pdf
None of those are "bass boost" headphones, but the two dynamics there are going to sound "lean" or "leaner" compared to the Orthos.
For comparison, here's a Grado:
http://www.innerfidelity.com/images/GradoSR325i.pdf
The Grado has the bump at around 10k, and very lean bass. The low-end extension available from something like the HD 650 is "more" - the HE-500 is "even more" than that. You're also likely to pull some stored energy out of the equation, but who knows - the HE300 is pretty scary:
http://www.head-fi.org/t/566929/headphone-csd-waterfall-plots/255#post_7791432
None of the Grados do that:
http://www.head-fi.org/t/566929/headphone-csd-waterfall-plots/435#post_7852298
Again, I'm simply stating that based on reviews I've tended to shy away from the orthos; for all I know they're the bees knees. What I can tell you though, is that I enjoy my 'stats, and for the money, wouldn't do it differently. Fiio also seems to make some fairly competent kit; so that's a plus too (I think you did good there). I think you'll be well served by your choices, but I expect things to be "very" to "extremely very" different coming from Grados. The Koss, compared to Grados, will still "bite" when they need to, but they don't "pierce" and can be very smooth when the media calls for it. I would consider the AKG K701 very capable of this as well, but you said $500-$600, and the K701 + amp should not cost $600 (then again, there's nothing wrong with saving money). I don't think you can really know if you like the HiFiMan purchase or not, without trying it out, is basically my point. I'm simply meaning to provide you some more options. If you love the HE-400, then you've hit a home run. If not, sell/return and try again.