You can try to revese the phase on the sub cause that is usally easier. YThat is just changing the + and - . IT could also be becasue both the mids and sub have some overlap and difference in location might make some cancellation. I guess the only thing to do in that cause is lower he ossover on the sub amp to slightly below what the audio control x over is at. Just some things you might want to fiddle with until you ge the 70 hz module. You can awlays put everything back