If you run both 15's all the way down, then add subs, it wont work. Unless the subs are designed to mate with the tops, which would be luck if it happened. I tried that for a few years, and had a huge bill at the reconer repairing my 18's in my subs(overdriving them trying to overcome the cancellation). If the tops and bottoms aren't tuned the same, the bass coming from each does nasty things, such as cancellation at certain frequencies. If you are gonna run subs, take the sub frequencies out of the tops, send only the sub frequencies to the subs. Used properly with subs, you don't want the bottom 15 in the SP4's doing anything. SP4's are a poor choice to run with subs for that reason. Basically what you would be doing is stacking dissimilar subs side by side, which rarely works.