On the studio live series 3, a high pass filter is available on every input channel, and all of the flex mixes. A high and low pass filter is available on the center bus, and a high pass filter is available on the LR busses. If you are using an aux (which has no low pass filter on the bus) for a sub out you could consider using the center out instead, with a low pass filter, as independent channels can be routed or not to the sub out. The center out also has independent output of the main LR busses. Since the center/sub out has a low pass filter you would be good to go with no crossover or drive rack in any 2 way or full range line array/sub situation. If you are using the center out for center fills or something else, use a matrix mix for that instead. It is far better for that purpose.
If you need help figuring out how to do this just search for high and low pass filter in the downloadable manual with a PDF reader like foxit that has a search feature. Hope this resolves your frustration, but what you ask for is already there. Make sure your console firmware is up to date as well.