It sounds like there is more space between each instrument.  Also it drives great high impedance headphones, like the AKG K601 I have, and couldn't bear with the STX. keep in mind though, that the sound is kinda bassy, and it tends to create a problem with headphones that already have a strong...

I have the STX and wondered about the same thing. Searched a lot of reviews, and finally bought the One.   There is no going back to the STX. The STX is so bright, it ends up fatiguing. The One is more analytical and more natural, I really enjoy the music.