well. my fiberglass box IS ported. and its recommended to stay within the specs RE gives ya, so i made it 1.2ft^3... but they say 600w RMS is the sweet spot... so i feed it 800 with the gains down a bit so my amp runs nice and easy.
the subs will have great SQ no matter if its ported or sealed, i DOUBT you will NEED 2 15's but thats up to you- you better have the room. id bet 2.5~3+ ft^3 for EACH. at least 5ft^3 ported, maybe get away with 4ft^3 sealed.
i would go ported, thats my prefrence. my single 10 doubled in output when i ported it. its amazing.
but they should be very loud none the less with 600 to each. and its recommended to stay at RMS with 2 subs, than go to peak with 1 sub. so your fine there.
port them to ~33hz, give them the required space, and float that 600w to them. make sure you dont blow\hurt your amp by running it at full power all the time, thats why i opted for the 800 instead of matching it for RMS. so i can do some nice burps if need be //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
um i think thats all. but be prepared to kill all the other systems in your town. and bump responsably!
if you install that system correctly, you should hit 145db with no problem. (i think- you might need a little more wattage)
any other questions?