shared ports, airspace

Running 4 Niche Audio NW15E, with 2- (1 per pair of subs @ 2 ohm) JL Audio JX1000d

Note: current setup is in my signature, sealed boxes, seperate at 2cuft per sub hitting clean at 143db

I want to go vented and I am doubling the sub power (adding another JL JX1000d)

The specs say to have 5cu ft for each sub and tune at 42hz with a 6"D x 3"L port

So my question's are, as below, because I trust your input. (btw, I do like these subs so please don't waste space saying "buy different subs)

With 2 amps would it be better so have

seperate boxes and ports for each sub?

1 box and port per pair of subs?

1 big box and port for all 4?

OR

Stick with sealed boxes and increase to 3.25cuft per sub which they state will get me +5db/66 (see specs as published below

 Tight Bass / SPL Extended Bass     

I tune the amps to JL specs with a multimeter and each sub will be at 500watts, and are rated for 600rms

I don't just want loud bass, because my highs are really good