The 339 is the smoother, warmer sounding and the more tube sounding amp of both. The Crack is faster, clearer and has a better defined bass. Bass depth is better on the 339.
The HD650 is great with both, If you want to clear up the HD650 go for the Crack, if you like it's darkness go for the 339.
I replied in the Crack thread. In regards to the LCD-2, the Crack won't drive it at all. You will need the 339 with 5998 like power tubes