Wet Sounds Help Please!

I currently have 4 wet sounds xs-650 speakers powered by an HT-6 amp. 2 in boat and 2 on the tower. I would like to stick with that one amp but i have channels 5 and 6 open and would like to utilize them. I dont want to sound like a cheapo but i would like to add just one Rev 8 on the tower. Would one Rev 8 sound decent? I am not looking to really hear it at wakeboarding lengths just want something to fill up those channels so they are not going to waste. That would fit good bridged on those channels power wise. Or do you all think i would benefit more from an XS10FA sub. I havent heard the free air sub and am kind of nervous that it wouldnt be much of an upgrade in sound. You're help is greatly appreciated!