passive crossover help Please

Hi, I currently am using a Magnaplanar X0-1 box as a highpass filter for my Acoustat electrostics. I am using a .0206 Wondercap cap. This is supposed to give an approximately 85 hz cutoff. I was using an AudioPro B2-50 sub with a variable crossover set at around 85-90hz. Well I got a new MBL sub and it only has 70 and 100hz. What size cap should I get to change the crossover to around 70hz? Bigger I guess? What exact size would you guys recommend? I haven't messed with this stuff in 15 years and don't remember all the formulas. My Muse 160 has an input impedence of 10K ohms. The XO-1 box is wired internally with one wire going from the input+ to a volume control(1 for each side), another wire going from the volume control to sub out+ and the cap is wired to the subout+ and the high out+. Everything is grounded together. This works and sounds fine with Audiopro but the MBL doesn't. I want to cross the Acoustats over at around 70. Any help or suggestions would be greatly appreciated. Thanks