We ran with our chopper door closed and chaff spreaders on and after some fine tuning and tinkering we were quite satisfied with the job of spreading our 9540 did behind a 40' cut .
We moved the longest fins to outer position and added a bit more adjustment to all the fins , modified the deflector and made sure the straw was being discharged from rotor into center of chopper . Prior to that ours was streaking the residue also .
You may need to install concave blanks or run some large wire on the right side to keep the trash up in rotor longer and off the chaffer , I am presuming you are running all round bar concaves in it presently .
If you remove the tail board you will see the deflectors benk8680 is referring to .
Ours was a 2012 9540 , it worked well for us , but along with twin stacks we also felt we were in the R & D department running a prototype machine .