Hi Paul,
thanks. Changing those values in 3dfim+ does not really change the results. I will drop using 3dfim+ for this purpose now. Instead, and since you said that 3dfim+ is not really designed for my aim anyway, I am going to compute the slope in Python based on the extracted power-spectra computed in AFNI. (Whatever the reason is that 3dfim+ "fails" on this particular preprocessing+dataset.)
Thanks for your help so far.
Philipp