The problem with your dilemma is that "loud" is just as relative a term as "sounds good". What is loud to me could be just kinda loud to you, same with what sounds good to me could sound like ass to you. Having actually played with and heard a 15" ICON on a PPI BK1300 I can say the ICON does fantastic in the output department, in the right enclosure of course. I can't see how anyone could be disappointed with what it can do in both departments actually. The two 12's have more cone area and with nearly the same xmax do have the potential of giving you more output. Don't discount the ICON though, the 15 I've played with can almost take your breath away at the tuning of the enclosure which I find pretty impressive for a single 15 on a little over 1k of power. Being that it's also the cheaper of your two options, I would suggest keeping the order as is and going with the ICON. Make sure you build the enclosure as closely to optimal as possible and find the best orientation to maximize output in the vehicle. Then, if it's still not enough you can sell it and pick up the GCON's.