If you want to do that, your best option is to use a processor that has a remote control.

Phoenix Gold used to make an EQ/xover with their LPL44 knob as an option for sub level out. The EQ215X. I have one of the Ti versions on the shelf, display model. Kinda big, but good sounding.

But if you're not looking to go that route, then you are better off IMO finding an amp with that feature.