To me, the biggest difference is bass and probably the soundstage if i understand the term correctly. HD650 feels very.. narrow? It's like the music is very close to ears (obviously not closed-cans close), but at the same time very distant if that makes sense? It's right HERE, but not really. Very strange sound. And it becomes even more apparent when i put on X2's. They sound really wide, i can't even tell where the sound is coming from, because it feels like sound originates somewhere outside the cups. The bass is superior, it had more punch where it needed. Also, X2's draws you into music, while HD650 produced music disappears. It's not engaging.

It seems to me that the X2's may seem to have more perceived bass response with the Modi2/Magni2 combo. I haven't heard the HD-650's driven by the Magni2 myself, but I have heard the X2's, and did notice their bass.

As for the soundstage, maybe just double check that the HD-650's are wired properly... the connectors on the left and right side at the bottom of the headphone cups should have "L" and "R" facing outwards. If one of them was inverted, it might cause them to sound screwy. It's a long shot, especially since the connectors are pinned in different sizes, so someone would have had to jam the connectors in improperly.

If everything is connected properly, maybe you just perceive the soundstage of the X2 to be wider... it's entirely possible. Soundstage with headphones can be a subjective thing, although there are generally accepted perceptions. I personally find the soundstage of the 650's to be entirely comfortable, they don't sound congested or flat to me, however I do know that things change depending on what is driving them. It sounds to me like you might just prefer the X2's.

You may also find that it takes a while for your brain to adjust to the differences between the headphones. I know a couple of people who have a preference for headphones that have a lower bass presentation, and higher treble as well. The 650's may not seem to reach as low or high, but the beauty in the 650's is resolution throughout their entire presentation. I'm not going to say to force yourself into preferring them, but maybe give yourself a good week with them exclusively, and then go back to the X2's and see how you feel. Or maybe you already have... my subjective opinion of the X2's was that they lacked the resolution I hear in the 650's, which translated to me as muddy/wooly/congestion - silly words, but ultimately I preferred the resolution of the 650's, their linear sound, their lack of fatigue, and quality of bass, vs quantity.