As a former Cannondale rider and steel frame rider I can say that the aluminum construction most likely doesn't have much to do with numb hands.
You need to reduce the pressure on the nerves running through your palms.
You can do this by varying hand positions frequently or trying to find the exact position that causes you grief.
You may also find that no amount of padding eliminates this.
But raising the bar level or higher than the seat can work wonders.