s10, 4 kicker 15 l7s

Current installation has 2 Kicker 15L7s in 6cu.ft each. Vented.

I now need to install another pair and build an new enclosure..obviously.

These are Kickers recommendations for the subs.
Sealed 1.5cu.ft (min)
Sealed 6cu.ft. (max)
Vented 3cu.ft. (min)
Vented 6cu.ft. (max)

So I'm kind of stuck with 3cu.ft being what I can get....But what would you guys recommend for enclosure? a 3cu.ft sealed enclosure which is the biggest this sub can go in if it's sealed. Or, would you do a vented enclosure at 3cu.ft.

SPL is the main aim here.

You don't mention this in your post, but have you considered how much room you have for your box? That is going to be the limiting factor for those 4 subs.

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VEHICLE: 2002 GMC Sonoma ZR2
Alpine CDA-7940
AudioControl EQT x2
JL Audio 1000/1
JL Audio 10W6 (originals) x3
Kicker ZR120
Kicker ZR460
Polk GXR-6 x4
Polk GXR-4 x2

Yeah man, thats taken into account. Thats why I say I've only got 3cubes per sub.