Best solution would be to buy a valve with an anti-cavitation trim.
THere is a fundamental problem with using anticavitation valves on raw water, which is this application.  Raw water can contain solids: Leaves, aquatic vegetation and fauna, and even the plastic ring things off of siz-packs.  None of those things are compatible with anticavitation trim, which has squazillions of 1/8" (3mm) holes.  Anticavitation valves make for highly effective, but expensive to clean, strainers.
Anticavitation valves in GLOBE VALVES (remember, this is a ball valve) for pressures below about 500 psi are "Cavitation Containment" trims, and they allow cavitation to happen but they direct the jet of cavitation bubbles to where it can do no harm.  Usually radially inward to implode on each other. 
Multistage trims in GLOBE valves are used for velocity control. Remember that the vena contracta velocity is a measure of how far the pressure is depressed.  Multistage valves are used for applications that cannot be addressed by cavitation containment trims, up to really frightening applications like supercritcal boiler feedwater recirculation (P1>4000 psi, P2<atmospheric)  Some multistage valves have almost microscopic holes (CCI) and some will pass 1/4-inch chunks (Lincoln Log)
You mentioned that this is a Neles ball valve.  Neles has a ball-valve trim option called the Q-Ball that has a series of plates inside the waterway of the ball.  The plates are punched with holes. The plates are parallel with the flowpath, and they align with the stem.  At small openings, the flow has to pass through all the plates in series, causing multistage pressure reduction.  As the valve opens, fewer and fewer plates share in the pressure drop chores, but then the pressure drop is decreasing because of line loss, pump runout, etc. With the valve at 100% open, the plates are edge-on to the flow and offer little resistance except to styrofoam coolers, tennis shoes, branches, and the like, which may be in the flow stream.  It will still strain out large objects in the flowstream, but if you overrotate it past 100% open, the flow goes backwards thru the holes and  sometimes the stuff can be dislodged.
Ast least one other vendor, NAF, offers a trim that is similar in principle.  NAF calls theirs the Z-trim and they market it primarily for noise attenuation of gas flows. The NAF trim arranges the baffles in an X in the waterway, so the wide-open flow path passes bigger pieces and there are 4 pressure reduction stages at low flows.

MORE NEWS