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The design tendency of the shocks are to "hold the car down" on vibration. I.e. as indicated the "rebound" is over-restrained compared to the displacement.
The spring is of a design such that as the spring is displaced, it presents non-proportional increase in resistance. (I.e. it grows stiffer faster than displacement) Thus the ride "stiffens up" under increasing loading/displacement.
This is the same principle used later (and with more fan-fare) by the designer of the VW suspension Ferdinand Porsche. VWs had the 60/40 built into tubular shocks and used "increasing rate" coil springs combined with "rubber donuts" to achieve a similar action.
I still suspect you may be "undersprung" and you're counting on the shock to take momentarily the load of compression. Bottoming out is only one aspect of ride and control.
You can test for "critical damping" which is the condition which Porsche and Ford before them were trying to achieve.
Given the usual number of occupants of the car in place, try to displace the car by jumping on the bumper thereby pushing the car down. If the return involves more than one "wiggle" (i.e. comes above the pre-displacement static point or passes through the static point more than once) you are under-restrained.
If it appears slow to return to the pre-displacement static point and never wiggles, then it is "over-restrained."
The ideal condition is the one where there may be little or no "overshoot" - this would be the critical damping condition. THIS is what you want and will afford the best ride and control.
Graphed below.
The beauty and engineering of the Ford/Porsche system comes in having the spring "tuned" to the loading of the car. More load yields a non-proportional displacement.
Joe K