It would involve a bit of work, but have you ever tried the "Herbie" system which works well for megaflows? Here's a few pics of mine in my external overflow box:
One hole is used as the main drain (the short one) while the other taller standpipe is the emergency drain. The main drain flow is adjusted with a gate valve until the water level is just at a trickle going down the emergency drain. Water return from the sump goes up and over the back of the tank.
Took a bit of adjusting, but now it's dead silent. All I hear is the hum of my skimmer.