The SF-2 is so cool! For one thing, it provides such a beautiful deep bass with the low-pass filter. Using one of the filters in hi-pass mode, you can get some great, sparkling highs that you didn't even know existed! (Well, you knew if your bass was an Alembic, but as someone else posted the SF-2 can bring out sounds in other instruments that are impossible otherwise) The band-pass filter lets you focus in, for example if there are some mid-freqs you want to accentuate.
I have a recording of my Rickenbacker 8-string that just isn't quite jumping out in the mix, so I'm going to experiment with routing the signal through the SF-2.