First off - just to keep things relatively simple & less confusing - I'd recommend putting the SF-2 in Stereo mode (via the back panel switch), and have the Filter Type switch (front panel) in Low-Pass mode on Channel A. Then, plug your instrument into the SF-2's front-panel input jack (providing a nice 10dB gain) and send the output from Channel A into the Effects Return jack of the F-1X. As such, this will by-pass Channel B entirely (one thing at a time!), and keep the patch cord mess to a minimum. Just so you'll know, the front-panel controls of the F-1X will operate exactly the same as they would if it's front-panel input jacks were utilized.
Now, for an initial flat setting on the SF-2, have Channel A's Direct Gain control set at 0, the Filter Gain control set at 6 (and a good place to leave it, unless you need additional gain), the Frequency control set to 6.0 KHz, and the Damping Ratio control set at 1.25 - now you're ready to experiment.
To roll off the highs, simply rotate the Frequency control counter-clock-wise to the desired point, and to boost the resonant frequency (or Q), rotate the Damping Ratio control clock-wise to the desired point - that's it!
If at any time you wish to compare your new setting to a flat (i.e un-filtered) tone without losing your setting, re-set the Filter Gain control to 0, and re-set the Direct Gain control to 10 - to bounce back to the filtered setting, simply return those two controls to the way they previously were - that's it!
One VERY important suggestion: to additionally minimize the confusion factor even further, start off with your INSTRUMENT'S on-board tone/filter controls set to THEIR flat settings.
Additionally, set the F-1X's controls as such: Bright switch down, Bass set at 2, Middle set at 10, Treble set at 2, and Deep switch down (the closest thing to a flat setting as can be found on the F-1X).
This way, you'll be able to hear the effect the SF-2 is providing without anything else additionally coloring the sound.
Happy hunting!