Assuming you are running the Superfilter in the effects loop of your F-1X, and not using it as a preamp... unplug it for a minute.
A little tip I got years ago, set the F-1X controls at 2-10-2 for B-M-T. Leave both 'bright' and 'deep' switches off. This approximates "flat".
That'll give you a starting point. Now, hook up your SF-2 again and tweak it back in. For whatever reason, I like to high-pass A, and low-pass B.
*Our Chief Electrician used to take pictures of settings in complex SCADA cabinets... it works for amp settings too.