I have a fiberglass box that holds 4 6x9's and two Alpine 4.150 amps. With the four regular 6x9's it gets really, really loud but you have to really turn up the volume in order to hear it very far. I would like 4 Wet Sounds 6x9's, but $1,200 is more than I want to pay at one time. Two of each, at least in my head, sounds like a good way to go, I would still have good volume close to the boat and now have good sound by the rider.
I actually have six Polk Momo's for the inside the boat, but haven't got around to installing them yet. I have a Signature Series and still have to cut holes in the interior vinyl to install them.
I think I actually saw another thread where someone else did the same thing and mixed two sets of 6x9's but can't find it.