I generally associate v shaped, bright, dark etc when there is a lack of detail or quantity in certain frquency range. For v shaped sound, classic example would be audio technica m50. Where there is lack of detail and substance in mid range.
I have seen certain reviewers describe perfectly balanced hps like hd700 as v shaped. I cant get aboard this description cause to my ears its mid range is not lacking in detail or substance. Its separated and instruments as a whole maintain some distance from listeners. True, after listening for some time to something like hifiman he400i, one finds it difficult to hear mids cause separation and distance. But once you adjust to its sound, it again starts to paint whole picture of the band again. Albiet at a distance from listener.
I see.
That's not what it is about, V-shaped or U-shaped is a jargon that defines how the Frequency response looks like on paper.
Having a U-shaped signature doesn't mean that the mids are bad, for example ie800 has amazing mids, yet it is extremely U shaped.
I'd say that HD700 is mildly to seriously V-shaped. This is not something bad, but something to take into account. I am what you'd name an audiophile and I love V-shaped and U-shaped signatures a lot
For refference, this is how HD700 looks like in its FR>
To my ears, HD700 sounded quite treble happy.
About ATH-M50, that is a headphone I dislike (some peeps might be amazed by this, but there are things I actually dislike out there). M50 has a slightly bass heavy signature, as you can see from its graph provided by Golden Ears.
The problem with M50 is that it sounds uni dimensional.
There aren't any measurements that can measure how much something will expand in the 3D space, at least for now.
At any rate, I have not heard HE-400i, but to most people something like ER4-XR would sound linear, maybe mid-centric / Mid forward, ie800 will sound U-shaped or V-shaped, and something like FiiO X5-3 will be extremely smooth/warm, while X5ii was a bit warm, but not necessarily smooth.
If you hear things differently, it'd be best to either try to understand why everybody names sound the way they do, so you can know what to expect, or find someone with similar tastes / opinions and discuss things so you have a similar vocabulary / expectations. This might / should help a bit (?)
I don't think I can provide any other help with this, understanding the audio vocabulary might take time, but I think that it is a part of this hobby.