I think the 4 15's on 4k rms would give you a little better overall sound, since you would be getting a flat response with lots of cone area and lots of power for potential output. Running them ported gives you roughly a + 3 dB increase in output, so at its tuned frequency, the subs that were getting 2600 rms ported, would sound like they were seeing 5200 rms as far as output is concerned. If you had the two 15's in the right enclosure and they were seeing a clean 1300 rms each, they might out do the 4 sealed 15's just a little. What subs are you looking into and what amps? Also is your electrical able to handle 4k rms? If not, it wouldnt take as much to be able to handle 2600 rms as it would 4k rms, so if power capabilities limit you, you may have to go with the 2600 rms set up.
Its really what you want sealed will give you a much tighter sound and give lots of output at higher frequencies but that also depends on the size of the box and the f3. But ported even with less power could get louder with lower frequencies. But I'm guessing if your asking what is louder you just want the loudest system for your money and it would be going ported. Making an electrical for 4k is not cheap. Your best bet is making an efficient enclosure to sound good and loud 2500 watts is capable of being extremely loud with a good design
get 2 D4 sa-15's and eather an sax-1200D or the saz-1500D.
unless you have a rather upgraded electrical. what are you working with can we know your battery's and wire used, BIG 3 done? do you have an high output alt? also what car is this all going in to?
i would say if you are running a real amp that puts out what it says for RMS any thing over ~1500 watts you are going to need more power to make that power.
i have been told that ~1500 watts is the upper limit of what the SA line will handle well, besides the SA-8. so lets say you wanted to run 3k total or 1500 to each thats ~300 amps of draw, well granted thats not a constant draw.
you may be happy with 1200-2000 watts and it would keep your build cost down. but i mean its your setup so in the end you will do what you want i can only give advice.
What he's saying though is your car has to be able to handle the current draw from the amp(s) you're running. In your car I think you'd be better off with the two 15s ported.
I dont want to spend no more than $350 a sub. Can I do three ported sa15's with subs up and port back with a port through the rear deck? i have 11.7 cubes before subs if I build the box in the trunk