for two of either of the subs i woundnt go any bigger than 8 cubes net personally i would try and keep it around 7 cubes. the sundowns would be sweet in 7.5 cubes tuned to 32-36hz just use 3 or 4 4" aeros or 2 or 3 6" aeros so you wont have to much port noise. you can prolly get away with 2 4 inch aeros but it would be tight and u would prolly end up with port noise
As of right now they are mismatched but I'm thinking if selling the AQ and buying another z v3 15 but I will have to save up a little more money.so I will probly just run one for right now