Maggie MG 10.1 How to cut below 85hz

How could I cut out everything below this so that my power amp doesn't duplicate effort with my powered subwoofer?

Doesn't your subwoofer have a high pass output in its crossover? You could use that, as the cheapest solution, but in most cases this is sonically just plain awful.
The second solution is to have a passive crossover made with first rate parts, which you would put in front of your Maggie inputs.
The most expensive solution is to get a real good active X-over, by which, experimenting with slope, cut-off frequency and volume you could perfectly attune the Maggies with your sub. However sonically also this solution will have an influence on the sound and often passive crossovers, if they are well designed, sound less detrimental. But since you would probably have to have it specially calculated and made for you, it is probably not cheap either. This is only my HO. I'm not an technical person, just talking out of experience. Regards
I had a similar problem with my Martin-Logan CLS II speakers. I purchased an electronic (active) crossover from Paradigm. It's a model X-30 and sits between your pre-amp and amp. You set the frequency for what goes to the sub and pick either 50, 80 or 120 Hz hi pass that goes to the power amp and your main speakers. There are several other companies making similar devices. It works quite well in keeping the low base away from the panels.