I find generic descriptions of sound profiles less than helpful.
As the dude of solder say, depends on which versions of each HP you are talking about but a quick look at the FR of each measured on the same rig speaks volumes.
SR60e
https://www.dropbox.com/s/o6l73ut2b02f4lc/Grado SR60e.pdf?dl=0
400i 2020
https://www.dropbox.com/s/ld9y8k3r67ikujw/Hifiman HE400i (2020).pdf?dl=0
Looking at the top left chart of each PDF and even ignoring the blue Harman target line, I can far easier relate what I see to how
@solderdude describes the sound than any attempt to align either to a letter of the alphabet.
Comparatively and absolutely the Grado has a massive upper mid/ treble boost and zero bass. The Hifiman is far more even. If you are used to/ enjoy that level of 2-4k action then yes, the 400i will sound a bit lifeless.
Neither look very much like a U, V, W or any other letter of the alphabet.