It is most likely the model headphones you have. Also, if your Zero hasn't hit the 100 hour mark yet, it is still maturing. Once you get the "Earth" on board and matured, you'll know if a change in headphones is in your future. Many of us swear by our HD-650's and their excellent sound. I think PP even sleeps with his on! He is so dedicated!! I just take naps with mine!!
BTW, the 1364 ahead of the stock 5532s in the amp are a very good balance match and quite tuneful! Though, I still find the 5532s are a bit lazy and opaque and lacking in the stage width that comes so effortlessly from KHA, whether via the tube pre-amp or direct to the amp and there is little sign of a midrange edge on Eva Cassidy's potent voice.