We're in the exact same position. Two years ago, I bought an HE-400 with a Magni / comparable dac to Modi and have enjoyed the heck out of the combo since. It just performs so well with edm and anything fast paced; no detail gets lost, even in the most crowded of songs.
However, like most, I find the high mids lacking. Every now and then I find myself looking to sit back and relax with some classical, acoustic / female vocals and, while it still sounds great with the HE-400, I find too much emphasis being placed on the drums rather than guitar / vocals. So, the upgrade bug bit me and the last few days I've been scouring the site for the best bang for my buck can that will do the job.
For a while, I thought maybe a new amp or dac would be exciting to hear, if not at least look at and know that I've got a nice piece of equipment. I've been dissuaded from that though, I think new cans, whether it be a side-grade or upgrade to the HE400 is the solution. Once I find the right headphone, I'll determine if my humble little stack is enough to get it sounded great. If not, I'll pick something out to do it justice.
Some phones I've been looking at recently:
Sennheiser HD600/HD650
Shure SRH940
HiFiMAN HE-500
AKG K7XX (although they do have a dip in the upper mid range, I keep hearing how great of a deal the massdrop is)
And lots of other stuff :S
I wish I had infinite money so I wouldn't agonize for weeks over making a choice lol