I have a very similar set up for the returns but I'm still using the dursos. My tank has 1 built in overflow with 2 returns and 1 output.
I made the dursos myself and played with them until I had found the source of the noise from the durso. The trick I found was to use 1" thinwall pipe. On the T that you connect the elbow to, don't drill a hole below the elbow. This I found was the source of the noise from the durso. You should only have 3 holes at 1/2" dia. each with the top of the hole level with the bottom of the side pipe from the T. You should only drill a 1/8-3/16" hole in the cap at the top of the durso. Into that small hole I fitted a small hose barb which silenced the small hiss I was hearing.
With (2) 1" returns exiting the same overflow box and the plumbing below the tank slightly different as it makes it's way to the sump, it was obvious that the water flow through both pipes was not equal by observing the discharges in the sump. The pipe that was flowing more was making more noise. The obvious fix to this was to slow that flow down and the way to do that was to place a ball valve in line. In order to regulate and balance the flow through both pipes I placed a ball valve in line with both returns. By slowing the flow through both return pipes it was like hitting the mute button on the TV remote. The trick here was to watch the overflow box and start turning the ball valves closed while also keeping the water discharges equal. Once the overflow started to rise at the top I simply backed the valve off just slightly. Then let the tank flow stabilize and tweak it again. I found that by regulating (reducing) the rate at which the water exitted the tank from a freefalling rate to a stabilized balanced rate that the noise was all but gone.
My rio 17hf is pumping totally unrestricted via 1" ID nylon tubing = 3/4" OD where it connects to the stub pipe below the tank at the bulkhead. From there it's 3/4" pvc with 1 elbow where it enters the tank through the overflow box.
There is absolutely NO noise whatsoever from the overflow. NONE NADA ZIP!
The only noise I had then was a slight grumble from the 1" pipe as the water accumulated at the ball valves. My cure for that was simple. Either re-plumb with thickwall PVC or use pipe insulation. For about $5 I bought 2 eight foot pieces of pipe insulation and with a steak knife I cut and fitted the insulation over the pipes under the tank.
Of course as you continue to silence one part of the tank the other components start to speak up. The next noise maker was the sump where the water was exitting the discharge pipes. By placing a T at the end of the discharge pipes with a small standpipe above the water line the discharges were silenced.
Then the skimmer return was left. The water falling off the return pipe was splashing so I installed a 45 degree connector and a section of scrap pipe so the return water was directed just above the water level in the sump which stopped the splashing.
My tank is 95% SILENT during the day with only the slight harmonic humm of the 5 fans in the canopy and that is only evident if you stand near the tank during the day. At night all the fans are switched off via timers. I only run the fans during light operation.
At night when all is quiet and the daytime noise of life is gone I can still make out a slight amount of grumble from the return pipes. I attribute that to my choice of pipe insulation as I chose the thinner type. This week if I remember I will pick up some thicker insulation and redo the pipes. I may even pick up some acoustic padding and line the inside of my lower cabinet sidewalls. This should kill any and all remaining water noise as my house is 100% tile and tends to carry sound well throughout the house. Yet, even before I change the pipe insulation and add the acoustic padding I would say my tank is 97% silent. My wife says she can't hear anything as it is, I guess my hearing is slightly better.
