Remember to start with the F-1X at the flattest setting of about 2-10-2 (bass-middle-treble). What are some of the settings you are using on the SF-2? are you using it after the preamp or in the effects loop? Using the bandpass mode and adjusting the Q to change the width of the band may be useful in not overpowering the highs or lows, depending on where you set your center frequencies. Make sure to keep the original sound high in the 3-way mixer if you are happy with it.
Speaking of that, does the bass acoustically give you the highs and lows you are after? Filters are subtractive, so they take unwanted things away. If your original tone is lacking in the upper frequencies, then you'll need to use a different EQ approach.
Also, what strings are you using? I cannot overemphasize the contribution of the strings to the overall sound.