What would be the difference between the 18-20" and 14-16"?that box is good. I always shoot for 18-20" of port per cube in my boxes from now on.
You'll run into problems with too little port area before you will with too much. On 600 rms nominally, probably half of that is what the sub will actually be seeing, so I say your design is just fine
On smaller power, you COULD go with less port area as your sub would be moving less causing less air to move. If you never plan on upgrading from a 600 rms amp then you could go with 14-16" of port. 16" minimum is what I would do. But what if you up power in the future? Might aswell go as big on the port as you can now just incase you plan on upgrading power in the futureWhat would be the difference between the 18-20" and 14-16"?