There's a thread on RC about the Tee in the sump vented to air. It was someone's anit-herbie thread. But anyways.. seems like it worked for some people, most it didn't. It can actually increase the air if you have enough flow from thew venturi effect. What your hoping to happen with the Tee is that your water is moving slow enough that the bubbles can rise in the flowing pipe out of the Tee. This is not common on most systems because of high flow.
I have my 2 duroso's set up in a semi-herbi method where one is gate valved and has 0 bubbles since it's mostly submerged(going to refugium). The other one pulls in LOTS of bubbles, but it's into my skimmer chamber so i don't mind.
The reason duroso's are quiet at the tank level is because they help the pressure differance(gurgling) by venting the top, adding air to combat the lost pressure from the water falling. A bubble free duroso isn't working as a duroso, it's just full of water.
So technically to make a duroso bubble free you have to balance your return pump/drain lines to have enough water to keep your drains full at all times(making it a herbie). The only issue with this is that if you drain line becomes restricted in any way, you will overflow your tank. That's why the herbie has the emergancy drain.
I threw a filter sock on my skimmer drain line, i wanted to avoid filter socks, but until i drill an extra hole in my running tank it seems like the only option to have 0 bubbles.
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