Purpleheart is liked by some, but disliked by many. It chrysals extremely easily - more easily than you would expect.
I think bamboo is seriously overpowering the purpleheart. Hickory is only mildly weaker than boo, so it is still overpowering the purpleheart I think. I would definately stay away from bamboo. Hickory may be used, but only if left very thin (under 1/8") and preferably trapped.
I would choose a lighter backing, such as Ash (a favourite), Elm or Maple. Then still, it may be adviceable to trap the back (make the back narrower than the belly). Leave the belly flat at all times, and keep as much of the limbs working as possible.