The 558s aren't exactly known for being the most balanced headphones; and along with your Vengeance 1500s, two headphones hardly make for "most headphones." Try something like a Grado. If the midrange on that still isn't clear I'd have an ENT check-up - best case scenario there could be some earwax build-up, so my ENT cleans out my ear every year while banning me from using Q-tips inside the canals as I just end up pushing it all in.* If your hearing is otherwise normal that should be all there is, but I just met a guy last weekend whose off-axis midrange hearing is shot. Meaning he has to be looking at you or your mouth is aimed into his ear canal for him to understand what you're saying, so if he lived a few hundred years ago, he wouldn't live to age 32 because he wouldn't hear the officers warning them of an incoming cavalry charge from the rear.
*He doesn't charge me a cent, he and his wife are like that with long-time patients (his wife was my pediatrician since birth), so I can confidently state that this isn't some scheme to make more money out of me