This is true for most motion controllers.
The RMC75, RMC150 and RMC200 can easily control when the natural frequency is 2.5 Hz. Maybe lower but no one has gone below 2.5Hz.
The House of Dancing Water in Macau platforms had a natural frequency of 2.5 Hz
https://deltamotion.com/peter/Videos/The%20house%20of%20dancing%20water%20-%20lift%20cues%20timelapes.mp4
The valve frequency and hydraulic cylinder frequency do not effect each other.
However I have Rexroth estimates that the combined frequency should be estimated as Hz_e=Hz_c*Hz_v/(Hz_c+Hz_v)
Hz_e is effective combined frequency
Hz_c is the natural frequency of the hydraulic cylinder and load
Hz_v is the frequency of the valve.
I have seen no mathematical justification for this formula but is seems to work when designing.
I will use this formula until I find a better one.