Does the Casablanca give you the choice of sending the full signal to the subwoofer channels (i.e. SLOPE OFF)? If so, then why not set the subs to receive the full signal and let the sub's filter let in what it needs.
But if you want to bypass the Thiels completely, start with the 80 hz frequency at 24 db. This steep roll off may result in a "hole" at 80 hz. Keep going down the slope decade until the sound is coherent, that is, the sound levels at that frequency are similar between the sub and the speakers. Ideally, they should be within 3db at 60,80 and 100 hz but not more than 6 db. A sound pressure level meter will greatly help. The bass should sound fast and quickly disappear - not resonating or reverberating.
If you go to 6 db, then the speaker woofer may overrun the sub and you'll get reinforcement (louder, bass boom, fat bass, etc.) at that frequency. Sorry, but this is probably a trial and error process. Also note that the Thiels have a 6 db slope - but that is with its own drivers and will probably not work so great with a sub at that slope setting.
BTW - make sure your speakers are properly set up before trying out the different slopes or you may not notice a difference.