From my personal experience its soundstage and imaging. My Salnotes zero sounds identical to my IE600 in treble extension and details but sounds very 2D.
My IE600 is able to give space between the instruments and I am able to pinpoint where each instruments are in the mix. Good height and width to the soundstage, very headphones like.
All my cheaper gear are unable to pull it off. Only the blessing 2 comes close but that's still high end. Soundstage is a very important factor to my music enjoyment. Its why my HD660, IE600 and B2 are able to make me tap my foot to songs I would never do on cheaper iems I have.
RSV is vocal centric from what reviews say. U12T looks like what you are looking for. U shaped signature.