As with all things audio, you have diminishing returns as you climb up. Sennheisers tend to have a very similar sound across their full sized open headphones. Grados tend to have a similar sound going up as well. There's a $1k difference across them. Whether or not the small changes in sound is worthwhile is up to the listener and the size of their wallet.
I was told to try out AKG K701 and especially Grado SR 325 when mentioned I find DT990 the most enjoyable. Any thoughts or experience to share?