Currently running JL 12w3's in a ported box @30hz with 900 watt rms. They get low and don't sound bad at all. But not real loud. Not looking to up my power, and am very curious to try a different brand. Thinking of the dcon 15's. But it will be easier to stick with 12's. So my question is will the dcon 12's give more output than my w3's on 900 watt rms in a 4 cu ft box at 30hz. Or should I just save for the 15's and a new box?? Hoping someone here had experience with the w3's and the dcon's and can chime in.

15s in a much bigger box will get you more output. What vehicle is this in? One dcon or icon 18 in a big box might do the trick too. What are the exact specs of your box? LxWxH and port size. What amp?

2014 kia forte sedan. I can't fit a MUCH bigger box. A little bigger I can fit. Box is 36w,14h, and I think 22deep. I think it's plenty big for 12's. Bigger than jl specs. And right in the range of the dcon specs. But obviously if I go to 15's I'll need a new box. Amp is jl jx1000/1

Port is 12.5 tall. It's a horn port. Flares out a little in front. And a lot inside the box.

Bigger box = more potential for different drivers to increase your output.

 

Always focus on the box first.  Can't consider 15's if you don't know that they will fit.  If you want output the first step is ALWAYS to figure out the max size box you can fit.  Once we know what that is we can actually help.