I tune my own suspension as well as a handful of local riders. Your understanding of the stock rebound stack is backwards. Rebound increases progressively as return speeds increase. Imo most likely your oil height is too high and low speed damping too light causing your harshness. Valve it stiffer and run the oil height around 325-330cc. Something to bear in mind when thinking about rebound is that whereas compression force is essentially an unlimited function of the speed of impact, rebound speeds will reach a limit when they balance with rebound damping resistance and spring force. There will never be any greater force on the rebound stack than the springs can deliver, while the compression force can be increased more or less at will by hitting the obstacle harder.