I've owned my pair of MDR-V6s for 4 years now, and I absolutely adore them. They were my entry into headphone collecting and headphone repair. They sound amazing, everything is crystal clear.
I've tried a lot of headphones as well. Things like MDR-V600, V700, V900/HD, SHP9600, Superlux HD681, Grado SR60, ATH-M50X. And now I've tried HD 599s and HD 6XXs... And my V6s STILL sound better than all of them. That can't be right.
Nicer and nicer headphones close the gap, but nothing does it like my 7506s and my V6s. Some have bass that sounds as good, some have mids that sound as good, some have top end that sounds as good... nothing has the whole package.
I'm just so confused... each time I go into a pair of headphones finally expecting to have something that makes me say "wow, this is better than the MDR-V6." And then I go back to V6s, and they sound better.
It's perfectly normal to prefer the sound of headphones you've used for a long time.
Also V6s are very capable in terms of plain ol sound reproduction so until you get into cork sniffy bits like height and air and slam etc there's nothing that should be outright lacking from the experience you get from them.
End of the day they are excellent headphones with a tuning you enjoy. You may find higher end headphones which improve certain aspects of the sound while maintaining a similar tuning, and perhaps improving on other aspects like comfort etc, but you will end up comparing a time-worn utilitarian studio tool to luxury items aimed at irrational hobbyists, so prepare for a big price jump.