Switching from the 668B to the HE-350 is painful. Sound seems muffled. Both are sparkly at the high end, but HE-350 is definitely more hissy. Mids feel distorted, and low bass pretty much disappears. Switch from HE-350 to 668B, and there's a marked increase in clarity. Separation between instruments seems to be better and the bass is hands-down better. The 668B is just way more fun to listen to.
The only thing HE-350 has going for it is better materials. It costs 2.5x as much so that's not surprising.
I sold these without using them at all. Unless you know you can put up with super-bright highs and a lack of bass, avoid.