I have a Mag 9.5 as well, but a durso on a 1.5" drain with 1.5" PVC.
My top cap has two holes, roughly 3/16" dia each. My suggestion would be to drill a second hole, start at 1/8" and go from there. Mine is not silent but is quiet enough for me.
My drain pipe is submerged in the sump, but is still louder than the Durso. The loudest component of my system are the drain pipe and fans on my ATI sunpower. The durso is 3rd in line, tied with the Mag 9.5.
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