It's funny but I had the exact same issue the last time I had a set of 650's. Basically, I received them used and they had the equinox cable installed. I wanted to compare with the cardas I had at hand, but the darn equinox would not pull out. These were the 2nd time I had 650's and the 1st time they pulled out very easy. So I pulled and yanked on it so hard that I totally stripped the wire, the pins stayed in the slot, and in my hand I had a half broken connector and some strayed wire. I tried it with the other side, and I made the same stupid mistake. I was able to use pliers to get out the broken pins, that at least allowed me to use the 650's, but my equinox was destroyed.
Then I plugged in the cardas, and when I tried to pull them out, same thing, wouldn't budge. In a fit of idiocy, I tried to pull them out again, and the same accident happened, it was so tight, the pins stayed in place and the connector and wire broke off from the connection (and yes I did it to both sides, "just to make sure" - man I'm dumb). Pliers again were used to pry out the pins so that a cable could be attached to the 650's, this time just the stock cable, as the overly tight connection already cost me a cardas and equinox cable.
Later I found out that there is a screw around the earcups that you can tighten or loosen up which affects the connection between your 650's and the cable pins. Supposedly the tighter it is, the better, kind of like binding posts on speakers. Long story short, just loosen up the screw that controls the connection, and you should be okay
