Stance vs. vibration
First thing to determine is what's the cause of your numbness - vibration, hand/wrist position, or weight on your hands. If you are not feeling a lot of vibration right off the bat, I would guess that's not the issue.
Look at your hand position when you're seated in riding position - are your wrists bent back? That's a classic position for causing nerve issues like numbness and repetitive stress. If that's happening, try having someone hold the bike up for you in the garage/driveway and play around with how upright you sit until you can hold your wrists in a more neutral position. If you can't achieve that with the current or rotated handlebar position, then risers or a new bar may be in your future.
If your wrists are in neutral position, then it may be the amount of weight you're placing on your hands. Again, sitting more upright may help here - do the same driveway experiment.