To add, there is nearly double the decibel differences between bass and mids on the X1 compared to the X2. There is about 10+db difference between the strongest bass point on the X1, and it's midrange, while the X2's generally falls between 2-5db at it's worst.
Also going by Tyll's graphs, the X1 and X2 are very similar after a certain point in the upper mids, which is why they sound alike up top, other than the 8khz notch, and stronger peak at 10khz. So if you play a lot of music that puts a lot of importance up there, they can sound quite similar.
Db = loudness, so when bass is quite a bit louder than certain ranges in the mids... it can warm up the sound considerably.
The X1 and X2 share a LOT in common, but it's biggest difference is between the meat of the sound between the bass up to just over 1khz. That is where the biggest change occured, and what gives them their different tonality. That and the 8khz pit on the X2 which shaves off ear fatigue, and more treble after 10khz, which gives it more air.