Fast replies - thanks guys. I have changed the crossover on the subs from 80 hz right through to 100 hz. I've also changed the crossover on the mains to see if that worked. If i move the mic it moves the dip a little but it's always there. I've tried moving subs, and mains but all that does is create another dip elsewhere as well. Actually it is a fairly narrow dip and the music sounds pretty good but it would be good to get this last little thing sorted
With the subs on their own they roll off at about 80 hz quite steeply. I've got two PB12+s and have one port blocked with each. I wonder if i should take the plugs out and go up to the 20 hz tune.