The JL Audio M6450 when rated like virtually every other amplifier, which would be at 14.4 volts, is 6x70 watts at 4-ohms and 6x110 watts at 2-ohms. This is "continuous power" and not "peak" or "dynamic" power.
Once put to task, at 660 total potential watts, the M6450 is within <less than> 1 dB of output as compared to some of the larger 4-channel amplifiers, if they are also rated with a 14.4 volt supply.
The JL Audio M6450 should be run in the six-channel mode at 4-ohms with a dedicated channel going to each of six in-boat coaxials. Remember that you still have a 660 watt power supply in reserve so the amplifier will be running conservatively, much cleaner and with extra dynamic headroom versus loading the amplifier down in a more compressed 2-ohm or bridged senerio. The amplifier will also run more efficient and cooler. This is a big amplifier so don't under-estimate it, especially when dedicated to purely a highpass application.
For a comparison example, a 4x150 watt at 2-ohms 4-channel amplifier would deliver the same 75 watts to each of the four cockpit coaxials and once you gain the bow speakers to a similar amplitude given the size of the bow you will be effectively running the identical power to these bow speakers as the individual cockpit speakers. So in a 4-channel amplifier you would be taxing the power supply more by running two channels at a 2-ohm load to arrive at 5 more watts per speaker when it would take at least 25 more watts per speaker to discern the most finite difference...in a perfect 'quiet' room.
So keep the JL Audio amplifier in the scheme and run it in the six channel mode. You have zero to gain by any other method.

David
Earmark Marine