They are not even close in sound and quality IMO
The X1 is thinner, more dry with loose bass and extended treble. They are warm but the mids on the X1 sound very uninteresting due to the thinner presentation. They do have good overall bass response for an open headphone but they don't have a very refined and controlled sound.
I would describe the X1 as more of a open fun headphonee.
The 650 is warmer, lusher much more seductive with a creamy thick presentation. It's more natural sounding and is very refined and controlled as you would expect from a Senn flagship. The treble is less bright by a mile with no peakyness in the sound and are very smooth. The HD650 has a full textured bass but it does drop off a tiny bit quicker then the X1.
The HD650 has what I'd call a fuller stage. It's wide but there is so much in it, if that makes sence? I do thinl the X1 has a more evident stage but it also has a much thinner sound.
The detail and resolution on the 650 is higher then the X1.
The 650 gives the LCD2 a run for it's money and I prefer it over the HE500. The X1 is not my cup of tea but I still had to judge it on it's technical ability and I found it performing on a similar level or maybe slightly higher the the DT990.
The cheaper K612 was a better sounding headphone that doesn't get as much praise.
Some may prefer the X1 over the 650 or 600 for certain tracks but switching over you can see the difference in musicslity, and refinement of the hifi Senn.