Whatever brand you choose for the tower, make sure it's pro audio equipment, and not car stereo stuff. Regardless of what people tell you, pro audio will
car audio when it comes to sound quality 75 feet behind a boat, at 25mph.
I went on a demo to hear the NVS Entity 808 tower setup, and it was just out of hand. It's four 8 inch woofers and two horns. You'd have to hear it to believe it.
I believe the website is: http://nvs1.net