Bruce is correct. I prefer to run the MB-8 on their own channels when running the Double Up, which is PRO 60 and MB-8. So you can cross over the MB-8 lower. Now since you have the 80's. You can run a 2 ch amp to the 80 and MB-8 combo since they are both 8 inch drivers and can cross them over the same. But since you already have a 4 ch amp, you should be good to go in any of the set ups.
Which Kicker amp do you have? I cannot find a 750.4 Only a 650.4 and 850.4. 650.4 does 120x4 and the 850.4 does 175x4.
So if you have the 850.4, you would have good power.
As for which speakers. Depends on your sound goals. The MB-8 is designed for those that want a system that has a ton of punch for surfing and party cove. So you get more mid bass in and around the boat.
If you are wanting a super loud system for the rider, then adding the additional pair of 80's or 60's would be the route.