Yes, i have another system. But with a different antenna. heading to the blind spot where my roof covers the reception.
Fact is that setting the gain to 49.6 was simply overloading the signal before using the filter.
I am not that critical on that. I have a good feeling and it was worth the 20 Euro.
Beside that the GSM signals went down clearly.
I ran the test as mentioned in the first post again, this time with filter.
Results compared. At least the filter does what it should do 
