Rather then 20th century class-A/B amps, I would consider the more efficient Class-D

Will you be keeping the stock sub? Once the speakers are chosen, the amp options fall in line. A nice efficient Class-D 4 chnl can power 2 pair of in-boats and a moderate 10" sub, but no room to add a 3rd pair of in-boats. For 3 pair and a sub, a 5 or 6 chnl is a great option. Quite a few proven efficient Class-D 5 and 6 chnl amps out there.

For tower speakers, I would suggest a dedicated amp. Being high on a tower as wells what we expect out of them, they will need way more wattage then the in-boats. Sharing 1 half of a 4 chnl with the in-boats rarely offer this needed power. For tower speakers, a 2 chnl is a great way to go as it will allow for a 2nd pair later. But, a 4 chnl run in 2 chnl mode can be a budget friendly way to power a single pair of tower speakers.