It's not flat from the upper midbass through the sub bass either. That's why I didn't care for them. I found the midbass to be rather lean. Music with a bassline or a beat was typically unengaging to me with K371.
I've not listened to the K371, but I know people who have them and love them, especially for the bass extension primarily, which is why I recommended them. If a person knows they like the Harman Curve they're an excellent headphone to get because they're so close to it....especially in the bass, and as long as they don't mind the strange dip in the treble at 4kHz, which can be EQ'd out to some extent. Horses for courses, but I think K371 would suit a lot of folks due to closeness to Harman Curve & combined with great bass extension. Recommending a headphone that is a close fit to the Harman Curve is a solid recommendation, just due to the nature of the research itself (far better than getting general advice from randos on the internet re their favourite headphone), but I'm not gonna get into a Harman discussion here....and out of the headphones that I'm aware of that fit the Harman Curve the K371 has the best bass extension so totally relevant to the OP.
I got a NAD HP50 (EQ'd), I suppose I could recommend them too, as they have great bass extension and are just a bit shy of my EQ'd Sennheiser HD600 in the vocals while exceeding them with the bass. I EQ'd mine (HP50) to the Harman Curve myself using Oratory's measurements because I found he had decided to EQ down 9000Hz below the curve because he thought it sounded harsh, but I felt it lacked some treble so I thought I'd take a clean sheet & just do the EQ myself to get it spot on (turns out I managed to use lower Q filters and also requiring less preamp, due to how I decided to position the target curve on the measurement):
Re NAD HP50, you can always use Oratory's settings though, but be aware don't just copy the preamp value because he calculated that wrong for this headphone, or he probably just did a typo or something re the preamp, it clips at his preamp setting, but yeah you can still use his NAD HP50 settings:
https://www.dropbox.com/s/mjbp2dau2o3m680/NAD Viso HP50.pdf?dl=0
https://www.reddit.com/r/oratory1990/wiki/index/list_of_presets
EDIT: re K371, I also did an EQ for that one the other day because I was bored (even though I don't own that headphone), so if the OP gets the K371 he can try this EQ (or anyone else), again it's taking Oratory's measurements and doing my own EQ to the Harman Curve. I managed to eliminate the 4kHz dip a bit better than Oratory in terms of it's about 2dB more accurate in that troublesome area whilst also using slightly lower Q filters. It's also more accurate in the bass....I think it's because Oratory limits himself to 10 filters so he probably decided to use them to concentrate on the treble. This EQ uses only 0.7dB more negative preamp than his too:
