Hard to get pictures due to limited access but here's a diagram of how it is set up.

http://hotimg25.fotki.com/a/70_70/254_23/Overflow.jpg

are there any reductions in the drain, at what level of the display is the water compared to the centerline of the drain bulkhead.

There were no reductions in the drain, all 1.5 inch pipe. The centerline of the bulkhead is probably around 4 inches below the level of water in the tank.

In any case, it is academic now as I have solved the problem. I built a pseudo Herbie overflow with 2 1" drains coming off the 1.5 inch bulkhead using a T. It's angled so the majority of the flow goes down one side of the T and I have a ball valve on it to control the flow a la Herbie overflow. The higher end of the T is the emergency drain and dips around via 2 90 degree fittings. It is virtually silent now. Yay!!!!

I'll try to post some pics later.

Finally getting around to posting a pic of my solution. It is a sort of Herbie going from a single 1.5" bulkhead to two 1" lines. It is nearly silent now and a huge difference from the constant sound of rushing water coming from the Durso.

http://hotimg23.fotki.com/a/70_70/254_23/Herbie.jpg