Hi, I've heard both, love both, for different reasons. In the right setup, the Ones will give you the recording studio, and feel more 'live'. and agreed with Nolitan, the bass is tighter. The Harbeth is more forgiving, less tight, beautiful mids, and can fill a larger room.
The Ones are trickier to get right, but when they're right, they're superb. Setup how they like it, I could easily live with either. And yes, the Ones need power.
Good luck.
Chas