Thanks to both Bob and Rick for your precious answers!
That is clear.
Actually, my previous idea was to filter the nuisance terms before the regression, as I tried to explain in the (B) method. Anyway, I understand that the suggested procedure is the simultaneous one (afni_proc.py -> 3dTproject).
I will run bandpass®ression in afni_proc.py, and then I will use the single subjects errts* datasets for the simple correlation (as suggested in step 2 in the afni_proc.py help page).
That should be ok.
Thank you!
Simone