Your not the only one. I've noticed that the variety of beans can make a difference. I just accepted it because I don't have a chopper, but it makes for a challenging spring with all those bunches out there.
Red Green, Check your email. I just sent you some pictures how I modified the curtain on my 2388s to solve that problem. It sure works good for me. Maybe you can make it work on your 1666.
Have you tried using some "disrupters" to break up the roped-up and tangled up stuff coming out of the rotorIJ Might make it easier for the spreaders to spread.
using old conveyer belting, I tried to make a "skirt" on my 1680 to prevent just this. I had the same problem. I bolted sections on sides and continued it on under the shaker extenson, connecting the 2 sides with a tarp strap underneath the extension. Really helped. I wish I had a digi cam to show you