Is your current box sealed?
If yes it is HUGE! You're looking at over 2.5 cubic feet. Even if there are 2 subs in there that is big box for 2 in a sealed. I recommend porting your current box.

But, when porting a box, don't simply place a pipe of any length in there. You will need to carefully tune it according to the speaker.

If your current box is for 2 subs I recommend installing 2 ports. Each port, 3" Diameter 15" long. THat should work well for you because each sub is seeing roughly 1.25 (or slightly more) cubic feet.

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