From the Hard hitter forum it seems that the dots who average a higher than 10% ratio of fumbles per tackle have tackling above 90 and strength very close to if not above 90.
One dude has 115 Tackling and his dot wrecks.
70 strength and 70 tackling are the kind of numbers you'll see on an average LB, not a hard hitter.
Yes, most HH's sacrifice speed, they usually have around 85 or 90 speed instead of the 110-115 you see on others, but if you're lined up in the middle stuffing the run you don't need that much speed. If you are going to dedicate your dot to being Hard Hitter you have to dedicate him to being a Hard hitter, and Tackling is one of the most important stats towards FF's. Followed by Strength.
Head Hunter also seems to be working well, but that's only a guess from watching stats as they change when players invest in it. I myself am a rookie to this and only have one HH, he's a Strong Safety and still in his infancy, so don't take my word for gold, but if you wanted to Force fumbles through Head hunter I would get speed and agility to an almost 1:1 ratio and invest in 1 or two pieces of First step AEQ or even buy a second bonus SA through the new training system rollout next season, I think that combo would work very well.
Tackling on Power doesn't seem to change much. Balanced is where it's at. If you would force a fumble on Power 9 times out of ten you would force a fumble on balanced, while tackling on power does increase your rate of missed tackles (we think).
Uhhh... I forgot where i was going with this.