so it says 4.5 cu ft recommended for ported box for an mj 18. how would it affect sound and output if i was to do say...6.5 to 7 cu ft. each? tuned to 30hz? would i get more bottom end? more output? what...

id try 6 that should give it enough room and it should perform well

You could try it, but there is a point when the box is too big and the sub thinks its IB. Wait until mark gets back to town to see what he says for sure.

has any one ever did a bandpass with th mj18?

I did it was LOUD did 148.4db's with 300Wrms....j/k but I would like to try it and see what's it's like

Bandpass is a trial & error type of enclosure...