Dsp is your best option. 40hz is roughly 27’ long so you could put your speakers in the 40hz null which would be 1/4th of 27’ feet… works out to about 7’. But this is only one boundary and all boundary sum…
bass traps would need to be super thick at 40hz. Like 3-5’ feet (just a guess I have no idea). Off loading to subs and using subs to control the room is another option.