Any speaker that is elevated up on the tower in open air, instead of down in the bathtub (cockpit), will automatically require far more power. Then add the fact that you are driving the tower speakers MUCH harder to project over a longer distance, rather than 5 feet away as you would with coaming speakers, and listening directly in the face of exhaust noise.
You need 75% minimum, up to the full thermal rating of the particular speaker for the tower. Since you're running the tower speakers in the highpass mode, I wouldn't hestitate to go a bit over the speaker's RMS power rating. In this case, with the XS-650, you are talking 75 min. to 100 watts per EACH tower speaker (not per set).
Ignore the false power ratings offered by source units. They're not even close to reality. Only look at honestly rated external amplifiers. Usually when you add an amplifier, you must also add supply cable and maybe distribution items.