They are Harman tuned, so most certainly not flat. I know Harman tuning is not flat because I have harman tuned IEMs, and they are far from it. Quite nicely elevated sub-bass, mids maybe dialed back a hair yet somehow vocals sound extremely lifelike. Treble is elevated yet not overwhelming.
Basically Harman neutral does not mean neutral in the way of HD600 neutral. Harman is a consumer-targeted thing, they are trying to match what most people enjoy the most.
I mean, to each their own frequency response curves. 
The Harman “curve” sounds pretty much flat to me except for the treble (always too much treble for me).
I would still prefer to try to point people towards the 177x instead of the dt 770 despite ot being more expensive due to the sibilance issues, higher ohm requirements, and just how much better the 177x sounds in comparison. I actually prefered the fidelio x2 hr(open back though) over the 770 in some cases but 177x was hard to be beat imo unfortunately its no longer on sale. I disagree on the signature of the 770 sounding more as a V and would definitely say its more U shaped as that signature to my ears doesn’t sound as sharp as a V would. I think the issue with the 770s is the ohms… it just doesnt sound good in low ohms so you need an amp to really get the most of it since youd want the 250 ohms. Least to my ears it doesn’t and like suggested you may find the k371 to be a better companion in this regard. Again though… this is my opinion on it.
You must not be all that sensitive to sub-bass then, I find them to have some of the most sub-bass out of anything I have. More than the Nighthawk Carbon. And I find the treble to be just right personally. It only stands out on poorly recorded songs to my ears.
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I got T50RPs. I like electronic music. Too much sub-bass never bothered me – not enough sub-bass and I hate the headphones.
(Too much sub-bass never bother me… well, unless you talk about old Beats headphones but that’s because they’re all muddy piles of sh…)
And see electronic is one of the places I think they shine, definitely a lot of sub-bass for that.
Human hearing generally needs more subbass than we think we need for a nice musical experience it’s just hard to really pull off in headphones especially mainly cause we work off of single driver per ears most of the time so things like muddyness is a concern where if you come form car audio and speakers it’s a non issue.
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