Automatic bass level control for the PA?

There are many solutions (like the driverack) that will do the trick. It also depends on your DJ mixer, for example I use the bass setting on my MCX8000 and it works fine.
Your current problem… well it is Behringer, do not expect it to work 😛
You can also easily try to solve it with a compression EQ (aka a compressor that will only do its compressor work on frequencies you select via an EQ like array).