Brandon,
Given that the 60 watt Polk tower speakers will be run highpass you can put more than 60 watts each to them. 90 watts each in our experience is about the safe maximum...and only if you take care of your stuff. You can do this with a number of two-channel amplifiers.
A source unit is not rated by the same standards that external amplifiers are. Figure any source unit that is rated at 4 times 50 watts to be about 16 real watts per channel best case. If you had a small and enclosed pickup truck cabin for example, you might be able to run HU power to four satellite speakers in combination with a 'sub lite'. But the second you go with a 10 or 12-inch subwoofer in an open field boat, you are going to have to drive the in-boat coaxials with external amplifier power.
Look to the JL Audio "J" series of amplifiers. A great mix of quality and economy. They have three monoblock, two and four-channel amplifiers in that series that should fit your application well.

David
Earmark Marine