Mike,
Are the 65 & 85 hz passive hp filters the only ones you offer?  I want to hook up a Force sub to my Tannoy D500 full range speakers.  I power them either with a Marantz 8B tube amp or a Marantz 250 SS amp.  The speakers go down to 40 hz +/_ 3db.  While I would like to relieve them a little of their very low end duties, I love the sound of my Tannoys and was hoping that I could squeeze out a few more hz from them before passing on the duties to the Force.  Perhaps by using a 50 or 55 hz high pass filter.  Does this make any sense from an audible point of view?  Would a 50 hz hp filter do little to relieve the strain on the Tannoys?  Is there any advantage in using the 65 hz hp filter over a 50 hz one? 
Thanks!
Brian