^^That's HE400's measurements, not HE400i, Here's HE400i for you. Roll of after 70 hz vs 50 hz of X2. HE400 bass is far better than HE400i by the way, nothing alike.
I'm fairly sure there wasn't an HE-400 remake in 2014.
Tyl has two links, the HE-400, and the HE-400i on his list of tested headphones. The HE-400i link takes you to the link I posted earlier.
Additionally, Tyl's own X2 graph which I posted shows a roll-off starting somewhere between 75-85Hz. Don't know where you're getting the 50 from.
Even if the roll of of the 400i's is at 70, which I seriously disagree with, that leaves the two phones nearly equal, no?
Additionally, it's important to note the derivative of the plot. While the bass may be quite a bit more boosted in the X2, the planar-magnetic drivers roll-off at a rate immensely smaller than the dynamic drivers. At one point, the average intensity of the 400i is going to be greater than the average intensity of the X2, which is why planar-magnetic headphones in nearly every single case have what listeners would consider better bass extension.
Don't become too attached to your gear. We're not here to market companies. We're here to tell each other what's good and what's not and offer a little insight into the wonders of equipment. The Fidelio X2's are some of the most fun and unique headphones on the market today, not to mention their fairly great value. While I don't believe the 400i is really comparable, it has its own traits to bring to the table, and we should respect the OP's curiosity by giving analytical advice. He should make up his mind based on his own preferences, and we're just here to offer our experiences.