need advise on how critical is box volume

[bkdent64]

hi guys, newbi here looking for box volume advise. I have a mtx thunderform DDXP20 box for a 97 dakota. the subs went out and looking to replace them. my question is how critical is the box volume, the internal volume is 1.8cf for both subs or .9 per sub. I was looking to replace them with rockfords R2D4-10 subs which require .59cf per sub or MB Quarts onx 254 which requires 1cf per sub, since i cant go up in volume for the MB's i was thinking that i could always come down in volume for the rockfords, if i did go down in volume what could i use in the box, i read somewhere that styrofoam blocks could be used, any advise would be greatly appreciated, Thanks

[frogcase2002]

is it a sealed box

[bkdent64]

yes it is