The difference in tap tone is probably due to the curvature.
It's generally a good idea to sticker wood immediately upon opening from the mail, or at least keep both sides exposed to the air during the first half hour or so. Some pieces will cup regardless, but uneven acclimation does it reliably.
Hopefully it will flatten out with weight and time.
If you plan to thickness with hand planes, I hope you're good at sharpening
Purpleheart is a hard, slick and chippy wood. Less-than-perfect blades will either slide helplessly over the surface, or rip and tear. It bends surprisingly easily considering its very high stiffness. When resin starts bubbling out the pores, it's hot enough.