Hope this helps.
Remove durso (don't throw away you might still need it), add a valve control near the outlet of the down flow pipe. Make sure all pipes are glued. This enables you to control the water level in the overflow box to the desired level and thus no more waterfall effect. When it reaches the desired level, tune the valve control till the right flow rate. This way you will not over flow the overflow box.
Observe the overflow box for about a day to see if the flow rate is same as your return. This is to prevent over flow. If you have two down flow pipes, use one for overflow. Extend the pipe slightly above the water level and below the edge of the tank of course. This way if there is any overflow due to blockage or slower flow rate, the 2nd pipe will there to take the overflow. You don't need a valve control for this pipe. Make sure the down flow pipe end is immerse in water if you want to elemenate the return sound at the sump.
What if only have 1 flow back pipe. You have to make sure that the flow back rate is always the same as the return or otherwise use back the durso method (that's why I mention keep the durso )
Have tried the durso but still have noise (flushing sound at times) and had a hard time controlling 2 durso. Most of the time only one is working. Have since given up on durso.
Thanks to Reefer Alanseah for his knowledge if not I will still have a waterfall at home.